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How to Increase Crash Risk, Fuel Consumption, and Exhaust Emissions

Leonard Evans

DIAMOND LANES make my frequent trips to California even more enjoyable. Accompanied by my wife, I drive around effortlessly in a rented vehicle. †My progress is not impeded by the majority of vehicles. They are prohibited from entering my lane because they contain only one person.

However, my 30-plus years experience as a traffic researcher specializing in safety convinces me that diamond lanes: 

● Increase crash risk 

● Increase fuel use

● Increase exhaust emissions. 

They increase crash risk in a number of ways. Greatest safety is achieved by minimizing relative speeds between vehicles; yet the very purpose of diamond lanes is to artificially generate a speed difference between vehicles in adjacent lanes. Vehicles traveling at high speed in diamond lanes are sandwiched between a concrete median barrier on the left and slow or stationery vehicles on the right. This is a dangerous situation that government policies should be trying to eliminate, not create.

Each of the 27 closest vehicles are likely using more fuel and emitting more pollutants than they would if all four lanes were equally accessible to all.  The unoccupied lane is available only to vehicles with passengers.  Not too many of them!  There are always some vehicles with passengers.  However, there is scant evidence that providing exclusive lanes for them materially increases their numbers.

Diamond lanes motivate eligible drivers traveling even short distances to weave through three lanes of slow moving traffic. Every lane change increases crash risk. Lane changing on typical freeways is prohibited at curves or other situations of elevated risk because of the additional increased risk in changing lanes. Lane changing is particularly hazardous when traffic in the lanes is moving at different speeds. In the absence of diamond lanes, a driver traveling a few miles would normally remain in a rightmost lane, which typically moves only a little slower than other lanes. Diamond-lane policy reduces speed in the right lane, while increasing it in the left lane. This increases the time saved by lane changing, thereby increasing the probability that drivers will chose to accept the increased risk to make them. 

Some drivers in diamond lanes end up traveling faster than they consider appropriate for themselves or their vehicles. This is their only socially-acceptable choice if they want to avoid the congestion-slowed non-diamond lanes. For compelling reasons left-exits from freeways are no longer built even if a right-exit costs more. Reluctant drivers should never be coaxed into left lanes as is done by diamond lanes. 

I could not fail to notice the dramatically higher density of skid marks in diamond lanes compared to in other lanes, or in left lanes of other freeways with which I am familiar. It seems to me extremely likely that people have been injured, perhaps even killed, due to unsafe situations caused by diamond lanes. 

When traffic is congested, the slower it moves, the more fuel it consumes. Fuel use increases rapidly with further reductions in trip speed. A scenario in which some vehicles travel in very congested and others in lightly congested traffic uses much more fuel than one in which all vehicles move in moderately congested traffic. By causing almost all traffic to move slower during peak periods than it would if all lanes were available, the overwhelming majority of vehicles consume more fuel. The vehicles in the diamond lanes are so few that even if they were to magically use no fuel at all, the aggregate effect would still be a substantial increase in overall fuel use. 

Exhaust emissions generally increase even more steeply than fuel use in response to vehicles accelerating after being slowed down by congestion. The large majority of vehicles undergoing larger numbers of changes in speed generate far larger increases pollution than any reductions associated with a few vehicles traveling at more constant speeds, or a few less vehicles in the traffic stream. 

The above observations would all be on target even if every passenger in a diamond lane were to be interpreted as one fewer vehicle in the traffic stream -- the theoretical upper-limit of any changes in traffic due to diamond-lane policies. To this observerís eye, the percent of multiple-occupancy vehicles does not appear obviously different from the percent in other states. Undoubtedly, some small fraction of the small number of vehicles in diamond lanes are carrying passengers who would otherwise be in separate vehicles. However, nearly all the drivers with passengers would have passengers even if diamond lanes did not exist. These drivers reap a windfall benefit at the expense of the vast majority of other drivers.

One cannot fail to be astonished that a democracy can support a policy that penalizes the vast majority in order to benefit a small minority. The minority enjoy lower fuel costs and shorter trip times. The majority pay increased fuel costs and suffer increased delay. However, all members of the majority and the minority are at increased risk of injury and death, and everybody breathes dirtier air. 

The fact that diamond-lane policy increases fuel use, emissions, and crash risk does not necessarily mean it is bad policy. Anyone going to church rather than lying in bed increases fuel use, emissions, and crash risk. The belief in diamond lanes may well have the important benefit of making the believers feel better. However, unlike church-goers, the overwhelming cost of the belief in diamond lanes is borne not by the believers, but by others.

On a personal note, on a recent trip to California I drove up and down Highway 101 at rush hour for a week in an 8-seat GMC Safari van, accompanied (most of the time) only by my wife. We are both grateful to the good people of California for providing us a fast free-flowing priority lane.  The tax-payers commuting to work to pay for our privilege were crawling in stop-and-go traffic in the other three congested lanes. We thank you, but we do question your common sense.

Leonard Evans

A shorter version Get Rid Of Diamond Lanes appeared in the San Francisco Examiner, 2002-09-27