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What's Up with Truck Speed Limits?


Anyone traveling on California highways has had the experience of getting stuck behind a big rig as fellow passenger cars zip by you in the left lane at significantly higher speeds. This phenomenon occurs because of the fact that the speed limit for trucks is in most places lower than it is for passenger vehicles. The differential speed limit on our state highways is relatively unique in California, as only six other states in the union similarly differentiate speeds between trucks and other vehicles. So why do California highway users get to suffer under this seemingly Byzantine experience?


The answer, as with most things, is complicated. First of all, many in the highway safety community believe strongly that heavy trucks are dangerous vehicles on the highway, and therefore need to move more slowly than the traffic around them in order to stay safe. Generally speaking, this harkens back to Newton’s Second Law, namely that mass times acceleration equals force. Not precisely equivalent, but relatively analogous, very heavy vehicles don’t have to be traveling nearly as fast to generate the same or more force as a passenger vehicle moving much faster. So the amount of force created by an 80,000 lb. semi-truck is going to be significantly greater than the average 4,000 lb. passenger vehicle at high speeds.


While the laws of physics are inescapable, they are not the only factor that determines how safe these trucks and other heavy vehicles travel on our shared highways. For example, commercially-licensed drivers have significantly more training than the average driver, are held to much more stringent requirements, and typically have more to lose if involved in any type of traffic altercation. Considering these and other factors, one must look beyond the potential impact of a heavy truck to really contemplate its relative safety on the road. In fact, modeling in a recent study by the University of California’s Institute of Transportation Studies suggests that increasing truck speeds to establish a 65 MPH uniform speed limit would lead to an increase in fatal crashes of 1.6% in urban areas and only 0.1% in rural areas. Given the results of this study, it isn’t clear that the differential speed limit is saving many lives.


There are other factors to consider related to speed limits on the highway, however. For example, one unknown, because it is very difficult to determine, is the number of crashes and fatalities that are the result of passenger vehicles taking undue risks to maneuver around the slower trucks on the highways. Presumably these types of accidents could be prevented if everyone on the highway was traveling at or near the same speed. In addition, there is a basic economic argument for increasing truck speeds. The goods moved by these trucks are integral to the state’s economy, and moving those goods collectively a little bit more quickly would lead to some increased economic output. On the other hand, there is an alternative consideration that isn’t directly related to traffic accidents – namely, the amount of air pollutants and particulate matter emitted by trucks as they travel up and down our state. These pollutants impact communities along highway corridors as well as contribute to global warming, and have been directly tied to thousands of deaths per year in this state. The fact is that emissions grow exponentially as truck speed increases, and so there is a public health argument for keeping trucks at a slower speed.


In the end, the question of differential speed limits is a policy decision that our elected officials need to determine. It is a great example of why we have policy debates, because it is a scenario of imperfect information and charged opinions – there is no real right or wrong answer - and in our system of government ultimately it must be sorted out through negotiation and compromise.

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