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|Copyright © 2004 by Leonard Evans
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14 How you can reduce your risk
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I open this chapter with a confession. I am in the weak position of a male obstetrician. While I study traffic crashes, I have never had the ultimate experience of being involved in one myself. This fact alone does not provide sufficient information to determine whether I know how to avoid crashing, or have merely been lucky. However, Table 1-1 (p. 13) shows that the probability that a random driver will have at least one crash in 48 years of driving is 98.4%. Some researchers would almost certainly at this stage invoke the phrase statistically significant. This has not been used in this book for reasons I have explained elsewhere. (p 377)
If merely lucky, perhaps I am foolishly tempting fate by my comments. At a White House news conference in 1960 a reporter asked President Eisenhower, "Sir, do you realize that on your upcoming birthday you will be the oldest President ever to serve?" Ike smiled and answered, "I believe it's a tradition in baseball that when a pitcher has a no-hitter going for him, nobody reminds him of it." (p 155)
In previous chapters many results relating to aggregate effects were presented, and interpretations offered of factors that affect overall traffic safety. Here we address the more personal question of what steps you as an individual driver can take to reduce your personal crash risk. The generally sensible advice in so many "How to drive safely" guides will not be repeated here. Rather, we offer general approaches and principles that may be less familiar, illustrated in some cases with examples from my own personal experience.
Behavior, not knowledge, is crucial
An informal "show of hands" survey I conducted at an international traffic safety meeting provided unmistakable evidence that some safety professionals had well above average crash rates.
Objective data on the behavior of traffic-safety professionals were provided by the measured speeds of Finnish road-safety researchers as they approached the venue of a national road safety meeting. Of the 13 researchers who could be tracked by radar in a 60 km/h speed-limit zone, nine exceeded 70 km/h, six of these exceeded 80 km/h, and three of those reached 90 km/h. The researchers' speeds were, on average, higher than those of the general public.
One of my highest risk trips was as a passenger in a vehicle transporting me between functions at a safety conference in a major US city. The vehicle was driven by a driving instructor!
I have not been much involved with the racing fraternity, and have chatted at length with only one racing driver - a great guy, as they tend to be. He had been involved in a fatal crash. Because there was no suggestion that he was legally at fault he was confident that there was nothing he could have done to prevent the death of the housewife who was killed in the crash.
Average behavior produces average crash risk
The low rates at which adverse events occur (Table 13-2, p. 340) repeatedly reinforce an impression that driving is extremely safe. Let us invoke the mental construct of a hypothetical average driver, who has the same 0.0858 probability of crashing per year as the US average, equivalent to one crash per 11.7 years (Table 1-1, p. 13). Such an individual has a slightly better than even chance of driving for 8 years without crashing (or 187,000 km, assuming 23,400 km per year). The copious feedback the average driver receives that average driving does not lead to unpleasant consequences cements the impression that average driving is safe driving.
However, in the same 8 years, this driver also has a more than 15% probability of being involved in two or more crashes (computed using
Eqn 1-1, p. 12) with l = 0.0858 and N=8). The average driver has no direct way of knowing that a natural and essentially inevitable consequence of average driving is involvement in one crash about every 12 years, or about 5 crashes in a driving career.
The probability that our hypothetical driver will experience at least one crash over a 55 year driving career exceeds 99%. Thus it is nearly certain that normal driving over a driving career will inexorably lead to a crash. A common reaction of drivers involved in crashes is to view them as rare unpredictable events outside reasonable human control, and of such a unique nature that nothing like them will ever recur. Yet, for our hypothetical driver, rather than being unpredictable, a crash is essentially inevitable. To realistically expect less than five crashes over a driving career, our hypothetical driver must adopt changed driving behavior that no longer matches the average behavior that copious direct feedback in the form of personal experience indicated was safe and appropriate.
For airline pilots it would be unthinkable to expect to crash about every 12 years, or for it to be inevitable to crash during a flying career. Yet flying is intrinsically more difficult and dangerous than ground travel. In my view, it
is within the control of the ground-vehicle driver to approach the same low
crash risks that commercial pilots achieve in flying. It seems pilots are fairly safe in road driving also, having the third lowest crash rate among 40 occupations examined.
Most drivers think they are better than other drivers
While most drivers would likely choose to change their behavior rather than accept one crash per 12 years, they do not feel at risk because they think that it is only other drivers who crash. One reason is that the vast majority of drivers believe that they are safer than average. Many investigations have come to this conclusion, but the question tends to be ill defined because the concept of average is left vague. Does it refer to those who live in your city, nation, or the world?
Study showing that most drivers think they are better than others
The best-controlled study consisted of two experiments in which subjects ranked themselves relative to others in the same room at the same time. One experiment had samples of drivers self-assess their safety compared to the safety of other drivers, while the other experiment applied the same approach to driving skill. The safety study used a group of 45 student subjects from a US state, Oregon, and another group of 35 students from Stockholm, Sweden. Each group was assembled in a room, so that every member was aware of the entire group. The participants were informed that, as there was bound to be a safest and a least safe driver in the room, all drivers could be rank ordered from the safest to the least safe. Each was then asked where they considered that they would be located in such a ranking.
The percentile at which each experimental point is plotted in Fig. 14-1 indicates the percent of the drivers who responded that they considered themselves to be safer than the drivers in lower percentile categories. For example, the values plotted at the median show that 88% of the US sample and 77% of the Swedish sample reported that they considered themselves to be safer than the drivers in the 0-50 percentile range. That is, they judged their safety to be superior to the median safety of the drivers sharing the room with them.
The reality data indicate what would be produced if every driver was given a list containing an objectively measured safety score for each group member, and had merely to report where his or her score ranked on the list. For experimental and reality data, the first point must be 100%, because all drivers must be as safe as, or safer than, the least safe driver. For the reality data each additional 10 percentile points along the x-axis must correspond to 10% of the population, leading to the linear decline shown.
By definition, 30% of drivers are actually as safe as or safer than the safest 70%, but 83% of the US sample and 51% of the Swedish sample considered themselves to be this safe. 60% of the US and 23% of the Swedish drivers considered themselves safer than 80% of drivers, whereas actually 20% are.
The skill experiment was conducted in the same way using different, but similarly recruited, students who were asked to rank their driving skills compared to the driving skills of the other drivers in the room. The safety and skill results show similar patterns of overestimation. Of the 36 experimental points in Fig. 14-1 all except three are above the reality line, indication that safety and skill were consistently overestimated. The three exceptions were all from the presumably more modest Swedish students not ranking themselves in the very highest categories; thus some necessarily ranked themselves lower than their true rank. Overestimation of safety and skill was far greater by the US than the Swedish students.
Other evidence of overconfidence
Australian drivers interviewed at home systematically rated their abilities as higher than average. The overestimation was greater for males than for females of the same age, the differences being large and systematic. The subjects similarly overrated their abilities, compared to the average, to drive safely after consuming alcohol. A study in New Zealand found that up to 80% of drivers rate themselves above average on a number of important characteristics, but also tended to rate themselves below 'a very good driver'. In a study of US students, 75% considered themselves to be safer than others, 89% thought they were more skillful, but only 54% thought they had lower crash likelihood. Young and old Canadian drivers systematically rated their overall driving ability, and their abilities at specific driving tasks, as above average. Another study further confirms that most US drivers rate their skills as better than average.
These systematic misjudgments are not necessarily corrected by personal experience. Responses of 50 drivers seriously injured in crashes and of 50 matched crash-free drivers were compared. When asked about how skillful they were, the two groups gave similar results, indicating that most of these early 1960s drivers, irrespective of crash records, considered themselves to be more skillful than most other drivers. On the other hand, another study finds that estimates of personal crash risk increase, and estimates of driving ability decline, with crash involvement.9 Drivers self-reported greater willingness to wear belts after being seriously injured in crashes.
Over 50% of all drivers do in fact have crash rates lower than the average. A parallel statement applies to many things, because real world distributions are more logarithmic than linear. If 99 average people and one sports superstar were in a room, 99% of the people in the room would have below-average income. However, the sports superstar would not unduly influence the median income, the income that was greater than earned by half of the people but less than earned by the other half. The distribution of crash rates cannot be symmetric, because no individual can have a rate lower than zero, or expressed differently, lower than the average value by more than the average value. However, a driver can have a crash rate higher than the average value by any amount. Thus the median crash rate (like the median salary) will always be less than the average. The focus of most of the studies reviewed is on medians. The public tends to think of average as meaning median, so the distinction is of little practical importance in the few studies that asked subjects to compare themselves to average drivers.
Why do most drivers think they are better than average?
Subjects asked to rate themselves on a collection of abilities or dispositions (gardening, clumsiness, house painting, intelligence, happiness, cooking, competitiveness, and musical ability), as well as safe and skillful driving, produced results generally similar to those for safe and skillful driving. Thus the driving case is part of a more general pattern. There is a body of research showing that people have a systematic bias in favor of thinking that their personal risk is lower than that of others for a wide range of hazards. While such optimism can contribute to an increased feeling of well-being, it encourages risky decision making in activities such as driving.
Because drivers think of themselves as safer than other drivers, according to the theory of cognitive dissonance, they are likely to interpret additional evidence as supporting this belief. Reports of fatalities, rather than inducing fear, tend to confirm perceptions of driving superiority, in that it is other people who are being killed. Most drivers have not been injured, so reports of serious injuries similarly confirm perceptions of driving superiority.
While fear is present in learning to drive, the goal is to eliminate it. Good driving is relaxed and confident. The longer one drives, the greater is the accumulation of evidence that all the really bad things happen to others. When drivers perform actions in traffic (say, driving too fast for conditions), and no undesirable consequences follow, the belief that the action is safe receives reinforcement, and is more likely to be repeated in the future. Indeed if a speed is perceived to be safe, why should a marginally higher speed not also be safe? The process has a tendency to generate increasing speeds until some incident, such as near loss of control on a curve, or a speeding ticket provides corrective feedback.
We receive day-to-day feedback confirming our belief that we drive better than others. We notice when other drivers maintain poor lane position, or turn corners with inappropriate trajectories, or without signaling. We are unaware when others are making similar judgments about us. As Robert Burns laments, "O wad some Pow'r the giftie gie us -- To see oursels as others see us!" The systematic bias in our perception of our own driving is somewhat akin to people's perception that they find more coins than they lose. They are aware of finding, but generally unaware of losing. Experience can be a false teacher.
Crashes and driver responsibility
Police procedures generally categorize drivers involved in crashes as being either at fault or not at fault. These designations are useful and necessary for administrative and legal purposes, and even on occasions for research. To assure completeness, I think it is helpful to place all the harm that can happen to drivers into one of three categories in order of increasing culpability:
2. Not at fault.
3. At fault.
Unavoidable harm in traffic
Terms like unavoidable crash occur with much greater frequency than events that are really unavoidable. The term occurs in innumerable formal documents and informal tales of woe. Some harm in traffic is genuinely unavoidable. There are cases of vehicles driving over bridges that collapse due to structural failure or earthquake, or vehicles being struck by falling parts from disintegrating aircraft. Drivers involved in such crashes are victims of random events over which they have essentially no control, given that they have decided to drive. There are no realistic changes they can make in their driving behavior to reduce such risks. In my view only a microscopic fraction of the risk a driver faces is due to events over which the driver has no control. Although the fraction is extremely small, the absolute number of drivers fatally injured in such events still far exceeds deaths from many other causes that command much more public attention and resources.
The tendency to seek similarities between the not-at-fault legal designation and the random events portrayed above flows from a human tendency to blame bad luck rather than ourselves when things go wrong. While I cannot make a bridge fall down as I drive over it, nor cause an aircraft undercarriage to land on my roof, I could easily and quickly become the not-at-fault driver in a two-vehicle crash if I wanted to do so. All I would have to do is to drive in traffic until followed by a tailgater (usually not very long!) and then select an appropriate moment to brake briskly. It is, of course, not the purpose of this chapter to provide instruction in how to produce crashes, but how to avoid them. However, if the reader accepts that it is possible through purposeful actions to become the not-at-fault driver in a two-vehicle crash, then it follows that it must be possible through purposeful actions to avoid becoming a not-at-fault driver in some two-vehicle crashes.
Not-at-fault drivers who claim that there was nothing they could have done tend to view their crashes as inflicted upon them by a traffic system over which they have no control. After all, they cannot control the behavior of other drivers, so there is nothing more they can do beyond driving legally and carefully themselves. Rather than accepting this, I believe that the individual driver can reduce crash risk substantially by taking steps to avoid not-at-fault involvement. Indeed, I consider that a vast majority (but certainly not all) not-at-fault drivers involved in multiple-vehicle crashes could have avoided their crashes by changing their behavior.
The first priority in avoiding risk, the easy one, is never to be at fault. Obeying traffic law largely achieves this. However, there are other actions that are not specifically proscribed by law that should be avoided. Commercial drivers (of trucks and busses) are subject to much more specific rules regulating how long they can drive without a break, and how much alcohol may be in their blood. All drivers should be sensitive to the increased risk of driving while fatigued or after consuming legal amounts of alcohol.
Speed is central. One encounters often statements claiming that some percent
of crashes involved speeding. Although the percent given is usually high, I believe it is an underestimation. Far fewer data document the role of speeding in crashes than that of alcohol. A post-crash alcohol measurement provides definitive evidence on alcohol in the blood just prior to the crash. However, after a crash a vehicle is stationary, and pre-crash travel speed is rarely available from objective measurements. Only a microscopic fraction of all crashes are shown by physical evidence to have occurred during travel at legal speeds. Although specific speed data are not available, I am convinced by the unstructured evidence that is available that the vast majority of crashes involve either violations of traffic law or clearly imprudent behavior.
The conceptual simplicity and frequent occurrence of rear-impact crashes makes them suitable to illustrate in detail the concepts introduced above. A rear-impact crash involves one at-fault driver, the follower, and one not-at-fault driver, the leader. This is legally how it is, and how it should be. It is the responsibility of following drivers to not crash into vehicles they are following, whereas lead drivers are entitled to slow down or stop, as the need arises, without incurring legal jeopardy. Close following, or tailgating, places a minimum of two vehicles at risk of crash involvement
The vehicle-following driver
It is the following driver's legal obligation to avoid rear-impact crashes by following in a legal, safe, and prudent manner. The extent to which this is generally not done is illustrated by the distribution of following headways on a US urban Interstate freeway in Michigan in 1978 (Fig. 14-2). In this figure the headway of a following car is defined as the elapsed time between the front of the lead vehicle passing a point on the roadway and the front of the following vehicle passing the same point. The two groups of drivers are identified based on driving records from police files. One group had one or more police-reported crashes in a seven-year period, whereas the other group was crash free. 27.3% of the drivers with crashes had headways less than one second, compared to 23.3% of the crash-free drivers. While the crashes were not just rear impacts, tailgating correlates with other risk-taking in traffic. The drivers had their crashes before being photographed tailgating, again showing that crash experience did not teach them to behave even as safely as average drivers. Many of the same vehicles were photographed on multiple occasions. Those tailgating on one occasion were likely to tailgate on other occasions, underlying the habitual nature of the behavior.
Relative to the reaction times mentioned in Chapter 8, and to the advice in most driver manuals that headways should be at least two seconds, driving with a headway of less than one second must be viewed as risky. Most drivers choose following headways less than the recommended two seconds, one study finding an average headway of 1.36 seconds.
Why do drivers choose to follow so closely? Tailgating becomes largely a driving habit, rather than reasoned conscious behavior. Drivers appear to do many things more for their own sake than for any utility benefit. Indeed, it is suggested that some criminal behavior is indulged in, not for the expected gain, but for the enjoyment of the activity. We have all observed one vehicle dangerously tailgating another on a stretch of multi-lane freeway containing no vehicles other than our own and the tailgating pair. The tailgater could often reduce his or her personal risk by passing, thereby also saving time.
Unlike many other forms of driver risk-taking, such as speeding, overtaking, or running red lights, tailgating does not save much time. If you ignore the question of other vehicles cutting into the gap in front of you, then following at a headway of 2.0 seconds instead of 0.5 seconds means that you arrive 1.5 seconds later. If the 1.5 seconds is critical, it can all be recaptured by, say, closing up on the vehicle in front just prior to exiting from the freeway. In that way the risk of a closer following gap is incurred for just a few seconds. Larger gaps do increase the probability that another vehicle will cut in front, but probably at less than one per 10 km of freeway travel under the fairly uncommon traffic conditions in which this is most likely. Even if a few vehicles do cut into the gap in front, this adds only about 2 seconds per incident to the overall trip time.
Drivers probably object to other vehicles cutting in front of them not because it delays them a couple of seconds, but because it is interpreted as some sort of personal affront, an assault on their manhood or womanhood.
Why are drivers so relaxed when tailgating? There are two reasons why many drivers feel so comfortable following at headways that unreasonably increase their risk of at-fault involvement in rear-impact crashes. First, the dominant cue when following is the relative speed between your vehicle and the one in front. This is normally very close to zero. There is zero risk of a rear-impact collision if both vehicles maintain identical speeds, no matter how high that speed is. The largely static visual impression in vehicle-following tends to lower awareness and concern regarding speed. If the speed of the vehicle in front changes suddenly, then the ensuing dynamical behavior of both vehicles is strongly speed-dependent, with the amount of energy available to cause harm even more so. The second reason why drivers are comfortable tailgating, and being tailgated, is that experience, that false educator, has taught them that it is safe.
Tailgating and platoons of vehicles
Tailgating can produce particularly catastrophic results when a platoon of many consecutive tailgaters forms. This is because of intrinsic platoon dynamics that may amplify disturbances as they propagate down a line of vehicles. If the lead vehicle of a platoon slows down gently and then regains its prior speed, the second vehicle may respond by slowing down more rapidly (depending on reaction time and headway). The third vehicle will then be confronted with a more rapidly decelerating lead vehicle, so that as we progress down the platoon, each driver produces a larger deceleration, until eventually braking capability is exceeded. For a sufficiently long platoon of vehicles with identical following parameters, a multiple-vehicle pile-up becomes inevitable.
Fig. 14-3 shows results from a mathematical model of a situation in which a stream of identical cars follow each other, initially all traveling at the same speed and separated by 40 feet. The position of each car is shown relative to the position of the first car (labeled 1), assuming that this first car continued at a constant speed. At time zero the first car reduces speed, but then returns to its initial speed. As we proceed down the platoon, each car approaches closer to the one in front, until car 8 crashes into car 7. One dear lady confessed that after seeing this result she always made sure she was never car number 7 in a platoon with more than 7 cars!
Figure 14-3. Mathematical representation of a stream of identical cars, initially all traveling at the same speed and separated by 40 feet.20 The eighth car crashes into the seventh as a consequence of the lead car (number 1) reducing speed at time zero and then returning to its initial speed 3 seconds later.
Naturally, her concerns should not have been so specific,
as Fig. 14-3 represents model output based on assumed
parameters. Different choices would have predicted different
specific outcomes. However, Fig. 14-3 does illustrate the
intrinsic instability of rows of tailgaters, which manifests
itself in the real world in the form of multiple-vehicle
pile-ups, sometimes with multiple fatalities. Crashes
involving the largest number of vehicles generally occur in
fog. Fog may encourage closer following in order to maintain
visual contact with lead vehicles, which will increase the
risk of multiple-vehicle pile-ups. Different effects may
also contribute, such as a lead vehicle suddenly appearing,
rather than being followed.
An individual driver in a platoon following at a large headway may damp out the disturbance so that no collisions occur. If you find yourself following many consecutive tailgaters, and are yourself closely followed, then adopt a headway larger than the normally safe two seconds. Drivers choosing safer headways for themselves may thus make safety contributions to the system, and thereby prevent harm to drivers who will be entirely unaware that somebody else prevented them from being involved in a crash.
How to reduce your risk of at-fault involvement
An individual driver can dramatically reduce the risk of being involved in an at-fault rear-impact crash by following at recommended headways rather than the shorter ones that experience leads us to believe are safe. The experience that the vehicle in front does not suddenly slow down must be replaced by the intellectual understanding that for it to do so is not an event of cosmic rarity. Everyone knows that there is a small probability that a small animal unseen by a following driver will suddenly run across the road. Everyone also knows that some lead drivers will brake sharply to avoid striking the animal. There is a high price to be paid for driving in such a way that when rare but entirely normal events occur, you crash.
Adopting a recommended headway will increase the probability that drivers cut in front of you. The resulting delay to you is minor. Ignore any perceived affront. If detached rationality cannot dispel such feelings, seek comfort in the confident, if less noble, expectation that the offending driver is likely to have more than the average one crash per 12 years. Let such drivers save their few seconds and have their fun - you know, even if they don't, that they are paying a far higher price for it than they realize. Recapture your few seconds by walking faster to your vehicle, thus improving your cardiovascular health, which will enable you to outlive the scoundrel who cut in front of you!
A case when safer following saves time. When a lead vehicle signals an intention to turn, many following drivers maintain a near-constant headway. This locks the trajectory of their vehicle to that of the lead vehicle, even though the lead vehicle will normally reduce speed substantially to execute a turn. You can improve safety and efficiency if you gently reduce speed as soon as the lead driver indicates intention to turn. When the lead vehicle completes the turn, your vehicle, although traveling slower than prior to the initiation of the turn signal, is still traveling faster than the turning vehicle, thereby saving you time, fuel and brake linings. By being further from the lead vehicle during its main deceleration, you have reduced your risk of striking it, or being struck by the vehicle following you.
Can technology reduce rear-impact crashes? Subjective estimates of head-ways can be unreliable. They may be influenced by the type of vehicle we are driving, as demonstrated in Fig. 8-1, p. 180. However, headways are easy to estimate - simply judge when a lead vehicle passes a feature on the roadway surface (a sharp shadow is ideal), and count seconds. Saying "one thousand, two thousand" usually works, but calibrate with a stopwatch when you are not driving. Check your following headway periodically, because it may drift towards the shorter values that experience falsely indicates to be safe.
More sophisticated devices to warn drivers they are tailgating have limited utility, and for the same reasons that apply to devices to warn drivers they are speeding. Speeding and tailgating are problems of behavior, not inadequate knowledge. The speedometer, and the simple method of estimating headways, provide the necessary information.
More sophisticated devices which automatically apply braking have been developed. I never felt more at risk of being in a rear-impact crash than when I traveled as a passenger in a vehicle demonstrating one such device. Extreme tailgating was used to show the device's capability. Even if the technology were perfect, the risk was would be high. If our vehicle were unable to match the braking of the lead vehicle, we would hit it. Yet we had no basis to be confident that the lead vehicle did not have superior tires or brakes than our vehicle. We would be especially vulnerable if a slippery patch of roadway surface was encountered at the wrong time. The threat that we would be rear-impacted by another vehicle far exceeded that of the very short headway drivers represented in Fig. 14-2. What might Fig. 14-3 look like for a platoon of vehicles equipped with such devices?
How to reduce your risk of not-at-fault involvement
When another driver follows yours too closely, you bear the risk of being involved in a rear-impact crash without enjoying even the modest time savings of the tailgater. In many cases you can reduce or avoid this risk by using a variety of techniques to discourage other drivers from following you too closely. To do so involves frequent use of rear-view mirrors, which is in general a good driving practice. If a vehicle follows mine too closely on a non-crowded freeway, I simply speed up to get away from it, or slow down to encourage the close follower to overtake. Some drivers attempt to intimidate drivers of vehicles they are following to travel faster by tailgating them. Respond to this threat as one should respond to most threats in traffic - by slowing down. The tailgater will eventually pass you and go bother someone else.
If traffic is congested, increasing your own headway and level of attention is indicated. It is not safe to keep your concentration and attention at its peak level at all times. This will produce fatigue, as it quickly does in novice drivers who devote all of their attention to the driving task in the pre-autonomous phase.
One situation in which being tailgated is particularly unacceptable is merging onto a freeway. Here you may have to abruptly reduce your speed if an attempted merge must be aborted. The tailgater's need to share attention between the merging and following tasks places you at high risk. If I am tailgated on a freeway entrance ramp I monitor the tailgater, reduce my speed substantially well before the freeway, and when a potentially acceptable gap comes along, accelerate rapidly, observing the tailgater recede into the distance in my rear view mirror. The time lag before the tailgater responds to the acceleration is readily observed, and provides an interesting indication of his or her likely time lag if I had braked instead.
Another effective way to deter tailgaters is to flash your brake lights. This is most satisfying if the tailgater then brakes. When in a particularly feisty mood, I have occasionally applied the brakes mildly, followed by acceleration, with most pleasing results. This approach should not be used if the tailgater is also being tailgated, as you do not want to precipitate a rear-impact crash. Earlier I mentioned that it was easy to become the not-at-fault driver in a crash. It is likewise easy to cause a crash for which you bear no legal responsibility and in which you are not even involved (the lead driver in Fig. 14-3 did nothing improper or illegal). If your actions convince the following driver that you are too crazy to risk following, then you have achieved your goal of increased personal safety while at the same time reducing the tailgater's risk. However, you may increase risk to another driver. I invariably observe tailgaters select new targets after I make it clear I will not allow them to tailgate me.
When being tailgated while enjoying the scenery on quiet rural two-lane roads, I have pulled onto the shoulder forcing the tailgater to pass. It is not uncommon for the tailgater to then proceed at a slower speed than that used previously to tailgate me, providing additional indications that the source of the behavior is habit rather than time savings. In congested city traffic, slowing down and using a sweeping rearward hand gesture visible through the rear window to invite a tailgater to keep further away usually produces the intended result. You occasionally get satisfying indications that the offending driver receives additional helpful comments from a spouse sympathetic to your views on the subject.
Approaching traffic lights. Drivers of vehicles struck in the rear while stationary often seem to think that it is self-evident their crashes were unavoidable. While a few are indeed unpreventable, most can be prevented. The risk of being struck in the rear while approaching a red light, or the risk of being struck while stationary at the light, can be influenced by your behavior approaching the light. Rather than proceeding at prior cruising speed and braking strongly close to the stop line, you can reduce crash risk by gently coasting towards the stop line while maintaining just enough pressure on the brake pedal to activate the brake lights. A vehicle is more visible when moving than when stopped. Basically, the goal should be to arrive in front of the stop line just as the light turns green, having reduced your speed as little as possible. This strategy also reduces vehicle wear and fuel use. Delaying the decision to reduce speed or stop increases the risk of being rear impacted. Red light cameras encourage the decision to stop after such indecision, and lead to increased rear impacts.
Sometimes you cannot avoid being so close to the light when it changes that moderate deceleration followed by a period of stationary waiting is unavoidable. For the period when your vehicle is the only one waiting, you are at some risk of being struck in the rear. Although there is not a great deal you can do, it is still worth keeping an eye on your rear view mirror. If any vehicle approaches in a threatening way, you will increase your conspicuity by flashing your brake lights off and on. This is because detection of dynamic cues is greater than the detection of the static cue of a constantly lit brake light, especially in peripheral vision.
System-wide effects. The system-wide effects of safe headways are not necessarily all positive. If all drivers were so selfish as to reduce their personal rear-impact crash risk by choosing two second headways, the capacity of freeways would decline. Indeed, freeway flow would have a theoretical upper limit of 1,800 vehicles per lane per hour. Incredibly, flows of 2,650 vehicles per lane per hour, which corresponds to an average headway of 1.36 seconds, have been recorded on a British motorway.18 As this chapter is aimed at the individual driver, the wise individual decision is to protect yourself by choosing a safe following headway of about two seconds. Let other more altruistic drivers assure high flow by following at headways that place them at increased personal risk but provide them little personal benefit.
Other traffic situations
I treated rear-end crashes in detail to illustrate principles that apply also in other situations. I particularly stressed that we are not obliged to become a helpless not-at-fault partner for risky at-fault drivers. Similar notions apply to other traffic situations.
Everyone knows that some drivers run red lights. If you are the first vehicle in line, it is prudent to glance left, and then right, before proceeding when the light turns green. The presence of stopped, or stopping, vehicles in each cross lane confirms that it is safe to proceed. Such increased caution is particularly important if you are able to approach the intersection without stopping just as the light turns green. In this case, by reaching the center of the roadway in less time than an initially stationary vehicle, you could surprise a driver running the red light.
In city streets do not place your faith in other drivers obeying stop signs, or adhering to right-of-way rules. Many drivers seem to attack stop signs at high speed, and brake at the last moment, even when they can clearly see traffic on the major road. This seems to be another driving behavior rooted in habit, rather than aimed at minimizing trip time. When I, and many other drivers, traveling on a major road see such a driver heading for our path, we slow down to see what develops. After the driver on the minor road makes the required legal stop, we regain our prior speed, and proceed normally. Thus aggressive drivers delay prudent drivers on the major road. But they also delay themselves, because they must wait until the delayed driver passes the minor road. It is another case in which more dangerous, aggressive driving is rewarded by increased crash risk, increased vehicle wear, increased fuel use, and increased delay. Pedestrians often similarly increase delays to themselves and others by standing so near the curbside that prudent approaching motorists slow down.
Many drivers tailgate a vehicle they desire to overtake. Assuming that the lead driver permits such behavior, it will generally increase overtaking risk. The maximum overtaking risk occurs when the overtaken and overtaking vehicle are adjacent, preventing the overtaking vehicle from quickly aborting and returning to its original lane. The time the vehicles are adjacent is reduced if the relative speed between the vehicles when they are level is increased. If the following vehicle starts very close to the lead vehicle, then the initial relative speed at the commencement of the overtaking maneuver is close to zero. If the following vehicle is further back, its speed can substantially exceed that of the overtaken vehicle by the time the vehicles draw level. Just prior to drawing level, the following vehicle can safely abort.
On relatively deserted freeways I often observe vehicles driving alongside each other on adjacent lanes. Such behavior increases crash risk for no apparent reason. If you find yourself alongside another vehicle, especially a long truck, then speed up or slow down. Be particularly wary of drivers who locate themselves behind your vehicle in positions in which they cannot be seen in your rear-view mirrors.
In general, keep as much space around you as possible. Major driving errors, skids, tire blow-outs, and such incidents are far less likely to lead to crashes if your vehicle is not close to other vehicles or objects. If a crash does occur, lots of empty space surrounding the vehicle is the safest occupant protection environment. In moments of lax concentration, drivers do drift out of lanes, change lanes without sufficient care, etc. Such threats cannot always be avoided in high flow traffic, but there is no point seeking them.
The above comments on methods for reducing risk all involved minimal, or no, delay. As speed involves a trade-off between safety and mobility, rational drivers might make different decisions on different occasions. Increased average speed can be obtained with the least increase in risk by focusing the speed increases preferentially on the least risky portions of the trip. Roadway portions with wide shoulders pose less risk than those with close guardrails, and guardrails pose less risk than solid structures such as walls, utility poles or trees. However, the basics should be kept firmly in mind. Increased speed increases crash risk, and, given that a crash occurs, injury severity increases steeply with speed (p. 209-217).
Consequences of rare events are highly speed dependent. The statement "There was nothing I could do - the child just ran onto the road" is rarely correct. Just because children are taught not to run onto the road without checking for traffic does not mean that the possibility that they will do so can be ignored. 295 child pedestrians under the age of 10 were killed in the US in 2002. Drivers should slow down substantially when driving close to parked vehicles or other objects from which pedestrians could appear suddenly.
Vehicle choice involves trade-offs between many factors, safety being just one. Few vehicles are purchased based on a single criterion, like the most comfortable, most fuel-economic, least expensive, most spacious, most fun to drive, best looking, or safest vehicle. Each individual will have a different solution to this complex problem.
I personally find it difficult to imagine benefits of motorcycling that are commensurate with the risk. But if people enjoy it so much, and do it knowingly, one must assume that they are making a choice that makes sense for them.
If you are in a crash, the vehicle attribute that most affects risk is mass. The heavier your vehicle, the more protected you will be. Although overall fatality rates are somewhat higher in SUVs than in cars of the same weight, the difference is due to rollover crashes. These are nearly all easily avoided at-fault crashes. The different fatality rates experienced by vehicles of the same type and mass manufactured by different companies are due mainly to the different types of drivers they attract, not the vehicles. Vehicles that acquire reputations for safety generally do so by attracting safe drivers. Crash ratings have no more than small effects on outcomes. The differences are minor compared to the effect of mass. A large, heavy vehicle may be the safest choice, but other vehicle attributes may be considered more important.
My own personal vehicle is a subcompact car with FARS weight 2,644 pounds (1,199 kg). With such a light weight, it is well represented in FARS! I like this car for easy parking and economy, and it is a pleasure to drive. Indeed, I am so delighted with it that it may be a long time before I replace it, which may be both good and bad news for GM. If I did replace it, it would be with a similar vehicle. I have little desire to own a Sports Utility Vehicle (SUV) or a large car.
Some people commend me for saving the planet by my vehicle choice. No such consideration influenced my decision, nor does the SUV have the enormous influence on national fuel use its critics claim. A year's driving of 12,000 miles in a SUV with fuel economy 21 miles per gallon consumes 143 gallons more fuel than a 28 mile-per-gallon car. One airline trip by a couple across the Atlantic or the North American continent causes substantially more fuel than this to be consumed. Yet no one has accused me of destroying the planet because I fly a lot. It is only when the cost of running an SUV or flying is increased by additional taxes on fuel that such travel will decrease. A campaign led by anti-SUV clergy asked, "What vehicle would Jesus drive?" This seems about as sensible as asking, "Where would Jesus vacation?" I would wager that those raising the question, joined by one mansion-dwelling broadcaster, were each responsible for consuming vastly more fossil fuel than those whose vehicle choice they deplored with such moral certainty.
Incentives to decrease or increase crash likelihood
The notion of incentives to decrease the likelihood of involvement in traffic crashes may seem unreasonable. After all, involvement in even the most minor crash is an extremely unpleasant experience, involving a ruined day, bureaucratic entanglements, and the loss of hundreds of dollars. A major crash may cause death. What penalties beyond these could possibly further motivate drivers to avoid crashes? Such an analysis fails to take into account the extent to which behavior is influenced by the perceived cost of crashing. Accepting that drivers would drive more carefully if their vehicles were wired to explode on minor impact implies that increasing the cost of a minor crash will reduce the probability of a minor crash (p. 350). My own risk-taking in traffic is reduced by the embarrassment cost I would suffer were I to lose the crash-free record that I was foolhardy enough to boast about at the beginning of this chapter.
Collision insurance increases collisions
If increasing the cost of crash involvement reduces the probability of crash involvement, then reducing the cost of involvement must increase the probability of crash involvement. Insurance sharply reduces the immediate cost of involvement in a specific crash by transferring most of the monetary cost away from those directly involved. The insurance industry uses the term moral hazard to describe insurance-induced changes in behavior, whether legal or illegal. It is clearly difficult to obtain empirical data on such matters, but there are compelling reasons to accept that they occur. I would certainly feel safer driving among drivers required to pay the full property damage cost of any crash in which they were involved rather than the actual situation in which almost all of the cost is borne by those not involved.
The non-purchase of collision insurance is a safety measure that slightly reduces mobility by encouraging more careful driving. But it saves a lot of money. Suppose you judge that your risk is average, and you possess resources that would allow you to pay for vehicle-repair or replacement costs without unbearable pain or financial ruin if a crash did occur. Under such circumstances I find it hard to think of any investment with an expected return approaching the investment decision of changing from purchasing to not purchasing collision insurance. Perhaps the decision to change from betting in casinos to not betting in casinos may have a better pay off.
If not purchasing insurance reduces your crash risk, the payoff is all the greater. If you are a safer than average driver, the payoff is dramatically greater still. My casual observation is that I and other motorists who carry no insurance beyond the legal minimum have crash involvement rates well below average, while motorists with abnormally high crash rates tend to shy away from even driving around the block without collision coverage. If you are a driver who does not crash and does not buy collision insurance, the insurance company will likely recoup your gratis premium by increasing deductibles and premium increases following crashes. So, not buying insurance yourself increases the cost of crashing to those who do buy insurance. This makes them less likely to crash into you, thus providing a small additional safety bonus from not buying insurance.
These remarks relate exclusively to the fiscal and safety benefits of not purchasing insurance to cover the repair or replacement of your own vehicle, provided you have the financial means to cover these costs yourself. Discharging obligations to others is a quite different matter, for which the law rightly requires every driver to carry insurance. Those driving illegally without insurance generally break other laws and are extremely high risk drivers, often at the fringes of society.
An experiment with a surprising result. While employed by General Motors I came up with what I thought was a neat way to investigate how risky everyday driving related to an individual driver's crash and violation record. My goal was to note cases of extreme risk taking (tailgating, speeding, weaving across lanes, unsafe overtaking, etc.) by drivers I observed during my commuting trip. I would use an on-board audio tape recorder to record the license plate of the offending vehicle together with my estimate of the gender and age of the driver, and record corresponding information for a nearby random driver judged to be of the same gender and similar age.
I was hoping to collect over a number of years a few hundred pairs of risky and control drivers, use the license plate numbers to identify the vehicle owners, and if there was no mismatch of gender or age, assume the owner was driving. The state files identify the driver license of the vehicle owner, so that the crash and violation records of the risky and control drivers could be compared. The aim was to apply the same procedure that led to Fig. 14-2 (p. 367), but for rare events that are much more extreme than occur within the time-frame of normal experimental observations. After collecting two dozen pairs I decided to check how things were proceeding by obtaining the vehicle and driver data from the state. This required a formal procedure to insure that no personal driver information beyond gender, age, and driving record was transmitted.
The data turned out to be useless for the purposes intended. Most of the vehicles with the high-risk drivers were owned by General Motors! They had accordingly no individual drivers associated with them. The drivers were participants in a program in which selected employees tested corporate products (at least, that is how the Internal Revenue Service interpreted it). If a participant crashed a vehicle, another vehicle was supplied, with the main costs being paperwork and embarrassment (unless there were police charges). The general demographics and other attributes of the drivers would suggest that they were likely to have otherwise been safer than average drivers.
An observational study in Britain found that people driving company cars traveled at a higher speed than those in their own vehicles. Traffic in the US and UK seems to be providing additional support for the more universal principle annunciated by Nobel Laureate economist Milton Friedman:
Nobody spends somebody else's money as carefully as he spends his own.
I believe that the original goal of my unsuccessful experiment is worth pursuing by a researcher not commuting to an automobile company facility. Collecting data is difficult and at times risky, as obtaining the license-plate numbers often required following a conspicuously high-risk driver, even a speeder, sufficiently closely to record the license plate number. I am not sure I would have done this in a vehicle that I owned!
Riskier driving is often indulged in simply because it is enjoyable. Driving is one of many activities in which pleasure and safety are in conflict. People in droves deny themselves the pleasures of ice cream, fried eggs, tobacco, and hard liquor in the expectation that it will keep them alive longer. Another approach to increasing longevity is to jog or lift weights, activities that would have astonished a laborer from an earlier age. Those taking such unappealing actions to extend their lives are never stigmatized as being more cowardly than others less afraid of dying. Important gains in life expectancy can be achieved by the rather modest behavior changes necessary to avoid participating in traffic crashes. Such behavior changes consume an infinitesimal fraction of the time required to jog, they are not all that unpleasant, and additionally extend the average life expectancy of other road users. Safe driving should, of course, be an addition to other healthy practices, not a substitution.
One individual driving more safely benefits not just the safer individual, but the entire safety system. In the situation portrayed in Fig. 14-3 (p 369), if a driver of any one of the first seven cars had adopted a larger headway, car number 8 would not have hit car number 7. The drivers of cars numbers 7 and 8 would never be aware of the trouble the other driver had spared them. As it is, the drivers of cars number 2 through 6 all contributed to the crash but proceeded unaware that it even occurred. A driver avoiding involvement in a not-at-fault crash is sparing some anonymous driver from being involved in an at-fault crash. Drivers refusing to be tailgated provide useful feedback to tailgaters that may tend to discourage their behavior. A driver approaching a traffic signal in a safe manner often prevents another following driver from running the red.
In a more general sense, safe and courteous driving encourages similar behavior in others. Risky driving is deviant from average, or normal, driving. If average speeds decrease, the most extreme speeders also slow down, but remain about as deviant from the norm as before. They retain their deviance, but at less risk to themselves and others.
In the broader sense, a nation's total traffic fatality count is determined by the summation of the risks taken by all of its drivers. A driver reducing his or her personal risk is making an important contribution to reducing a major national problem.
Summary and conclusions (see printed text)
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