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Killing speed
  1. I B Pless, Editor

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    “Speed kills. Kill your speed”. This clever message has appeared in the media in the UK for several years. Other countries use similar phrases. Although brief, all the essential ideas are represented. They state the problem and offer a solution. In this respect, it is a model safety message. But the question to be asked of it, as of any other preventive measure, is: does it work? Is there good evidence that drivers have slowed down?

    I acknowledge being preoccupied by speeding cars in part because I bicycle to work. Speeders are part of every trip and I occasionally see the grim consequences of their behaviour. In vain, I shake my fist and shout at the violators but it seemed that writing this might be more effective and safer. After deciding to do so, however, I discovered that I had been scooped by an editorial in the BMJ.1 The author does such a good job of summarizing the main points that I quote him extensively and shamelessly. Moreover, if memory serves I have editorialized on this before and may at times be quoting myself. In any case, I make no apologies for doing so again, such is the importance of this problem. It is the same everywhere in the world, affecting all age groups. (I do apologize, however, for drawing almost exclusively on material from the UK. It just so happens to come in a readily available form.)

    The problem

    Numerous studies show the relation between car speed and injury. The “kill your speed” message originates with the estimate that “the chance of a pedestrian [or cyclist, presumably] being seriously injured or killed if struck by a car is 45% at 30 mph and 5% at 20 mph”.1 Other studies indicate that someone hit by a car travelling at 35 mph is more than twice as likely to be killed than if the vehicle had been moving at 30 mph. The original data are cited in a UK road safety strategy document issued in 1997.2 Another frequently cited study suggests that for every mile per hour reduction in average speed there is an 8% reduction in fatal injuries.3

    Although to the best of my knowledge neither study is based on solid empirical data, their credibility rests on elementary physics. A 2000 pound object moving at 20 mph hits a stationary object with a decelerating force of 40 000 Newtons (mass × deceleration). In this it is assumed that what is being hit—a pedestrian or bicyclist—is, effectively, stationary. Highway crashes, however, rarely involve immobile objects. Even hitting a barrier or median is not necessarily as grim as might be expected because modern highway engineering incorporates some yield in these structures. Many other motorway collisions involve striking a car in motion, and except for head-on crashes, the direction of collision makes the physics complex. The point is that injuries to most occupants in highway collisions are often less severe than those involving urban pedestrians. But even on highways speed is the main predictor of what happens in a crash because, other than monster sports utility vehicles or trucks, most cars weigh roughly the same. Thus, the bottom line: the greater the speed, the greater the damage. Beyond any reasonable doubt, speed kills.

    The solution

    We need not spend much energy (no pun intended) on the second part of the message; it is a direct corollary of the first part. If speeding is the problem, driving more slowly is the solution.

    How good is the message?

    It seems unlikely that in this instance as in so many others, the message alone persuades drivers to slow down. Recent studies show no indication of a decrease in average speeds either on highways or in urban areas. The UK Kill Your Speed Campaign web site states that seven out of 10 drivers regularly break the 30 mph speed limit.4 It adds that two thirds of all “accidents” in which people are killed or injured happen in areas with a 30 mph limit. These were figures before the campaign began, but recent reports indicate little change. A comprehensive study into driver habits indicated that 10% exceed the 30 mph speed limit by at least 10 mph, apparently undeterred by the risk of a £40 penalty. Most of the 3.2 million motorists recorded during a 28 day investigation at 10 sites drove above 35 mph, the minimum speed that usually triggers a fine. Some drivers were recorded by automatic detection equipment travelling at more than 70 mph in a 30 mph area. Clearly, the message alone has not been effective.

    Making the message work

    An important exception to this depressing picture is found in communities that have adopted 20 mph limits. A review cited by Pilkington1 showed that 20 mph zones had average speed reductions of 9.3 mph and that this was associated with a 60% reduction in traffic “accidents”.5 Moreover, he notes that the 20 mph policy has wide public support, at least in London.1 So the question now arises: why is this not mandatory country-wide, and indeed, why doesn't every country impose such restrictions?

    I have a hunch that part of the answer is that few drivers associate the word “speed” in the message with the usual speed at which they travel. Some may not even associate it with doing 40 mph in a 20 mph zone. The average motorist thinks speeding means hitting 100 mph on a highway posted at 65 mph. However, many more lives would be saved by lower limits rigidly enforced in urban areas than if an equivalent effort were to be focussed on highway travel. There are two reasons: first, the victim in urban areas is usually an unprotected pedestrian or bicyclist (like me); second, there are more cars and more potential victims in urban areas. This fits with the late Geoffrey Rose's arguments that population based strategies for prevention are more effective than those focussing on high risk groups. A general slowing of traffic everywhere, but especially in urban areas, would have greater benefits than trying to catch drivers racing along highways at 100 mph and more. This is so because cities include much more of the most vulnerable population. UK estimates are that eight times as many people die each year on rural roads as on motorways. The same ratio applies when using accidents per vehicle kilometre travelled.


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    Experts continue to debate whether slowing highway traffic is possible, highly beneficial, or cost effective. Many studies suggest that slower speeds resulting from the oil embargo in the mid-70s reduced fatalities. Others (for example, Gullon6) sharply disagree, arguing that the decline in deaths had more to do with the business cycle, possibly mediated through driver distraction. However, the evidence for 20 mph limits in urban areas, combined with speed cameras7 and harsh penalties, is compelling.

    Equally persuasive are empirical data showing the devastating consequences of speeding. A recent study using a case-control design, shows that “68% of casualty crash cars were exceeding 60 km/h compared to 42% of those not involved in a crash”.8 The authors conclude that 46% of speed casualty crashes probably would have been avoided, or reduced to non-casualty crashes, if none of the case vehicles had been travelling above the speed limit. Stated differently, they note that “in a 60 km/h speed limit area, the risk of involvement in a casualty crash doubles with each 5 km/h increase in travelling speed above 60 km/h”. (And, in a fascinating aside, the authors equate the risk of speeding with that of alcohol, and ruefully observe that the former only receives a $110 fine while the latter gets a $500–$900 fine and automatic license disqualification for at least six months.)

    A study in the US examined child pedestrian injuries on residential streets and concluded that after controlling for many other factors the essential variable is speed.9 Estimates from this study are that an increase in average speed from 20–30 mph is associated with 7.6 times the risk of pedestrian injury.

    Political considerations10

    For a while it appeared that the Transport Minister in the UK was poised to introduce a policy of zero tolerance of speeding. He then backed off under pressure from various sources, including the motorist lobby. Although not as pernicious as the gun or cigarette lobby, it is just as powerful in most countries. For example, the Automobile Association said it would support strict enforcement of the law (only) if road signs made it obvious that the limit was in force. A senior Tory stated, “Motorists who are generally law-abiding will not take kindly to longer journey times and a guaranteed fine every time they accidentally creep over the speed limit”. The government was also reportedly nervous about being seen to be “anticar”. “John Prescott would do well to remember that the frustration caused by slow-moving traffic can make drivers angry—and this can lead to foolish decisions. It would do nothing to improve safety on our roads”, said the Shadow Home Secretary.

    Another excuse for not following through with the crackdown was that police lacked the resources to enforce these limits that are “too low”. But new policies to permit police to keep some of the share of ticketing would increase resources and allow them, for example, to invest in roadside cameras.

    Bottom lines

    A spokesperson for the Institute of Advanced Motorists, says speeding is endemic. “Most people see it as a minor infringement. The government's efforts to improve road safety need to render speeding as antisocial as drink-driving is now”. The Home Office Minister agreed that “30 means 30” and added that the best way to ensure that the law was respected was to enforce it.

    It is estimated that road deaths and serious injuries could be halved if 20 mph limits were introduced in built-up areas but a spokesperson for Transport 2000 said the measures would not go far enough and the director of the Pedestrians Association, said: “The question of speed limits is too important to be left to local authorities”.

    The last words are those of the Prime Minister: “We have the second best safety record in Europe, but the fact that 15 children are killed or seriously injured on our roads every day is simply unacceptable”. He added: “We will target accident hotspots around schools, improve driver training, and how we teach our children about safety on roads. Our 50% child casualty reduction target is a tough one. It will take a huge effort to meet it, but I believe we can”.

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