Speed Limits: Safe Driving – or Driver Mis-Education?

What are US speed limits teaching US drivers?

Posted Jul 30, 2016

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Source: JS image

This is a stretch of divided highway in northern Virginia, two lanes on each side of a wide median entering a gentle downhill curve.  The far two lanes (in shade) are the ones I am interested in.  As you can see, the speed limit is 25 mph, which might strike you as low for a 4-lane divided highway like this in a relatively rural area.  And you would be right, because the speed limit on the stretch to the right in the picture is 35 mph.  In other words, the limit changes from 35 to 25 without any real change in road conditions. 

Perhaps this is this a “speed trap”?  Drivers pass the 25 mph sign going at least 35 mph and downhill.  The chance that a driver not familiar with the area will fail to slow down is obviously quite high.  Look behind the white car on the left.  It is the policeman in the picture below, tracking a car with a speed camera. So, whether this is a trap or not, speeders are certainly punished.   

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Source: JS image

These pictures illustrate three things about US speed limits:

    1. They may change, even when road conditions vary little.
    2. Often the maximum safe speed will be considerably higher than the posted limit.
    3. The cost of failing to obey a speed limit may be substantial.

Features one and three train drivers to look attentively for speed limit signs.  This is distracting since the signs are unpredictable – drivers cannot be sure when to expect a change in speed limit so must look for signs all the time.  And the signs are always off to one side, deflecting attention from the road ahead.  Feature two trains drivers to mistrust their own judgment of what is a safe speed, since the limit speed is often very different from what driving conditions indicate to be a safe speed.

Speed traps aside, US speed limits are usually set to match the road conditions.  The problem is that this is impossible.  A limit correct for a clear, sunny day with little traffic will be too high for a foggy day and busy traffic and much too high for icy conditions or snow.  A compromise speed will be too low for good conditions, too high for bad.  In other words, a fixed limit cannot be correct all the time and will be wrong much of the time.  Short of an expensive automated system that assesses traffic, weather and road conditions in real time and adjusts the speed limit accordingly – in other words, a system which duplicates the driver’s own judgment –  absent such a system, speed limits that constantly vary in an attempt to match road conditions are more likely to mislead than inform. 

What is the alternative?

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Source: JS image

The picture above shows a sign common in the UK but unknown in the US.  They usually appear as you are leaving a town.  The sign says that the town limit, usually 30, has ended and the limit appropriate to the type of road you are now on – the National Speed Limit (NSL), as it is called – is in force.  Note the word type.  There are only four types.  This section from the UK Highway Code, the instruction book for drivers, describes the types:

UK Gov image
Source: UK Gov image

In other words, in the UK, no attempt is made to match speed limit precisely to road conditions.   

The picture above the table will be shocking to US drivers.  It shows that the NSL speed limit on this tiny (the figure in the background gives the scale), basically single-lane road is 60 mph.  Having driven on the road, I can tell you that no one – no one – goes at that speed.  Unless they are drunk or deranged – in which case they are likely to ignore any sign.  Drivers adjust to the road conditions and go at 20 or 30.

There is yet another arbitrary feature of US speed limits: they are often are often set low, in total defiance of road conditions, just to satisfy the preferences of local residents.  The preferences may be simply aesthetic – or may reflect the fact children sometimes play in the street.  Whether the motive is good or bad, the method – written instruction, a sign – is always bad, for the reasons I just gave: distraction, and training drivers to look for signs rather than at the road.  

Better methods are available.  Speed bumps are one possibility, but they require maintenance.  They need to be visible, even at night and not too steep lest they damage vehicles. A better method is to use what are called chicanes; the picture shows an example.  Alternating intrusions into a two-lane road force traffic to periodically give way so speed slows naturally.  No sign reading required!

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Source: JS image

Overall, the Brit system obviously makes more sense.  It guides the driver rather than attempting to control him at every instant.  The result: safer driving conditions and a lower accident rate than the US.  

It will be hard to change the US system of traffic control.  It is both decentralized and longstanding.  Many aspects of traffic-control policy are set at the state and local levels, even though common sense as well as elementary psychology urges the need for a consistent system across our nation.  Traffic-control policy is one of those few social functions -- like national defense and inter-state commerce -- that require national control.  Current practice, which is highly decentralized, has evolved over many years and changing it will be not just expensive (though less than another small war, I daresay) but, more important, will involve shifting attitudes long held and hard to change.  Perhaps if people understand the problems with the present arrangement, it will be easier for the politicians to actually do something about it.  That, at least, is my hope.