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Speed Kills - the Big Lie?

Are the messages about speeding true, or just so much propaganda?

By Dennis Jensen

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We have all heard the TV commercials and read the ads in the papers. "Speed Kills!" "Don't fool yourself, speed kills", or "Subtract 5 to save lives." Very succinct, to the point, and, what is more, seems to make sense. The fines, heavy police presence, the speed cameras - they're all there to save lives and reduce the road fatality rate - so we're told. Furthermore, the message goes, the speed limits have been very carefully set after a full and detailed scientific and engineering analysis, as well as community consultation. Well, the last is certainly true, but what about the first two?

The purpose of this article is to analyse whether the speeding message we are getting is true, or whether it is so much propaganda.

Setting Speed Limits

International studies going back decades have clearly indicated that the single best method for setting speed limits is to use what is known as the 85th percentile method. Basically, what this means is that the speed limit is set at or slightly above the speed at which 85% of free flowing traffic is travelling. [Ref 1, 2, 3]

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One of the earliest studies conducted that showed this trend was one conducted by Cirrilo in the mid-1960's. [Ref 4] This study showed that the fatality rate was lowest at the 85th percentile speed; this speed tended to be 10-15 km/h above the median, or 50th percentile speed. A study by Solomon around the same time also found that the crash involvement rate was lowest around the 85th percentile speed. [Ref 5]

If this is so, why is it that the authorities are not using this method in the setting of speed limits? The short answer is money - the governments are effectively addicted to the revenue that speeding tickets bring in. It is a case of accepting the status quo - with revenue continuing to come in from speeding fines (and this amounts to tens of millions of dollars) - or raising taxes to bring in the extra revenue. What the authorities don't appear to be looking at are the lesser costs that will be associated with reduced fatality and injury rates, if speed limits were correctly set.

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So, I hear you argue, how do you know that speed limits are not set at the 85th percentile? Speed limits are certainly not set at the 85th percentile in Australia, and this is clearly demonstrated by government statutory bodies' (such as the RTA) own documentation. The RTA, in its document Speed Problem Definition and Countermeasures Strategy [Ref 6] acknowledges that median speeds are above the speed limit.

In fact, according to their data collected from many sites, the 85th percentile speed is anywhere from 7-23 km/h above the speed limit!

I reiterate: the 85th percentile speed is in excess of the speed limit at all 52 fixed camera sites!! That is to say, the speed limits, using 85th percentile methods, should be between 10 and 25 km/h higher than they are at all sites where the RTA has set up permanent speed cameras. [Ref 7]

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Their stated aim is to reduce the 85th percentile speed to a speed below the speed limit. They certainly do not appear to have done their research very well, if their aim of reducing the fatality rate is, in fact, correct. International studies have repeatedly demonstrated that changing speed limits have extremely little effect on both the prevailing traffic speeds and crash involvement rates. This is very well demonstrated by exhaustive research conducted by Parker et al, in 22 states, 100 sites measuring the speed of 1.6 million vehicles. [Ref 8]

Looking at RTA data, the other issue that is clear is that the speed limits have very little effect on the actual prevailing speeds that motorists drive at along the road. The RTA has reduced the speed limit on so-called residential roads from 60 km/h to 50 km/h. The result was nowhere near that which might be expected; the 85th percentile speed went down by a mere 0.77 km/h, and the median (or 50th percentile) speed reduced even less - a mere 0.55 km/h. In fact, these reductions are so small that they are virtually non-existent; they are in what is called the statistical noise.

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These pitiful results in terms of actually altering the speed at which drivers travel reflect what happens on a worldwide basis. [Ref 9]

In general the speed limits are set too low and speed limits do little to nothing in terms of actually affecting speed behaviour. In fact, it is interesting to realise just how ill-informed the authorities are on this issue. On the NSW webpage, they state that 42% of crashes have speed as the causal factor, not realising that this means that speed caused the crash. What they really mean is that speed is a factor in 42% of crashes (as defined by them).[Ref 10]

Interestingly, British studies indicate that speed is a causal factor in only 6% of crashes, and contributory in only 7.3% of crashes (TRL Report 323). It should be noted that, in terms of fatality rates, Britain has the best record on the planet! Seems they have been doing something right. [Ref 11]

Why 85th percentile?

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As stated previously, international studies have shown that the crash and fatality rates have a minimum at the 85th percentile speed. Looking at the RTA data given above, and thinking about the speeds at which people are actually booked for speeding, it becomes evident that those drivers that are by and large being fined are those same drivers that international studies have shown to be the safest drivers on the road!

But why is that? Surely getting people to drive more slowly would reduce the energy that needs to be dissipated in the event of a crash? The clue is in the last six words of the previous sentence, "in the event of a crash". Simply, the speed at which you drive is irrelevant unless you have a crash.

Pretty straight forward!

It must be said, however, that the simplistic energy argument is correct if a crash does in fact occur; the higher the velocity difference between vehicles crashing, the greater the damage. From the foregoing, it is clear that what occurs is that the drivers driving at 85th percentile speeds are having significantly less crashes than drivers who are driving more slowly.

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If you look at the graph of fatality and crash involvement rates shown above, you will notice that drivers in the bottom 30% of driving speeds are just as dangerous as those in the top 5%. This really is counter-intuitive; you would certainly expect those that are driving so slowly to have a lower crash and fatality rate. The reason that this occurs is that there are far more factors to consider than simple energy or braking distance considerations.

If you think about it, if you drive more quickly, the amount of time you will spend on the road will be reduced. This will lead to a reduced exposure to risk; for example, if there are a certain number of random occurrences that occur per hour that could potentially lead to a crash, then reducing the number of hours that you spend on the road reduces the number of those random events that you are confronted with. Many of those random events are associated with traffic density - the higher the traffic density, the higher the rate of the potential crash events. For this reason, if everyone increased their speed, they would be exposed to less risk, and there would also be a lower traffic density.

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There have been some studies that would appear to indicate that this is not the case. For example, there is the very well known study by Kloeden et al that is frequently quoted in the media; you know the one, the report that states that the chances of being involved in an injury crash double for every 5 km/h increase in speed.

Let's have a look at the figure to see if this actually makes sense. Firstly, you will note from the graph that the error bars are such that there is, statistically, no meaningful difference in crash rates up to 70 km/h! Secondly, the 85th percentile speed appears, from the data, to be about 65 km/h (or just less), so it appears that this is an example of where the speed limit is not too badly set. So, the point that Kloeden makes is not so strong after all! Lastly, it has to be said that Kloeden's assertion that vehicles travelling over 88 km/h in a 60 km/h zone having an infinite probability of having a crash are completely ludicrous! [Ref 12]

The Effect of the Elimination of Speeding on Free Travelling Speed Casualty Crashes

Nominal Speed Speed Range No. of Cases Relative Risk Expected Cases* % Reduction in Crashes
35 33-37 0 0 0 0
40 38-42 1 1.41 1 0
45 43-47 4 0.94 4 0
50 48-52 5 0.62 5 0
55 53-57 19 1.01 19 0
60 58-62 29 1.00 29 0
65 63-67 36 2.00 18.0 50.0
70 68-72 20 4.16 4.8 76.0
75 73-77 9 10.60 0.8 90.6
80 78-82 9 31.81 0.3 96.9
85 83-87 8 56.55 0.1 98.2
- 88+ 11 infinite 0.0 100.0
Total   151   82.0 45.6

* Assuming all relative risks for speeds above 62 km/h are reduced to 1.00

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Overseas studies show the lie of increases of speed limits resulting in increased fatality rates. The US raised speed limits from 55 mph to 65 mph in many cases in 1987, and then in 1995 the states were again allowed to set their own speed limits. The head of the NHTSA, Joan Claybrook, predicted 6400 extra deaths. Those extra fatalities did not eventuate, as many studies, such as one conducted by Stephen Moore of the Cato Institute, clearly demonstrated. [Ref 13]

In fact, have a look at the fatality rate curve of recent years for the US. I challenge anyone to point to those years that speed limits were increased.


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There are other factors that also play a part in this total risk analysis. People can only maintain a certain concentration level for a certain period of time, after this, the concentration is reduced, and the mind tends to wander. For this reason, the final period of driving prior to arriving at the destination is inherently more dangerous than the beginning. If the period is reduced by driving more quickly, the final period will remain the most dangerous period on the road, but the riskiness will be somewhat reduced.

Of course, there is still the increasing energy and braking distance side of the equation that does negatively impact should a crash occur. However, the issue is a complex amalgam of all of these factors, and simplistic arguments are found wanting. In fact, there is no real theoretical way that would be conducive to analysis in terms of minimising fatality rates.

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The simple fact is, experimental evidence is the only real way to determine what is the best speed at which to drive (and hence set the speed limit). This speed is the 85th percentile speed, the speed at which the authorities are fining drivers for speeding, and the speed that the bureaucracy chooses to blithely ignore in their quest for revenue. The police are being used as glorified tax collectors. The majority of the police are genuinely attempting to reduce the fatality rate themselves, but are unfortunately being given the wrong information due to the government needing its revenue "fix".

In Western Australia, where I live, the same pattern that has happened elsewhere has occurred, in that the concentration on speeding has increased markedly. More fines were dealt out, a far greater camera presence was evident, a slight reduction in the number of drivers exceeding the threshold used was found (whatever that means, the threshold may have changed), but a higher fatality rate resulted between 1998 and 1999. [Ref 14]

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From all of the above, it is clear that the emphasis on enforcement of speed limits is misplaced. It would be far better for the effort currently expended on speed limit enforcement to be placed on improved driver training and the targeting of negligent and/or dangerous driving. Rigid speed enforcement is great for revenue raising, but it does essentially nothing for the road fatality rate.

Next week: a very different view from the Road Accident Research Unit of The University of Adelaide





4. Cirillo JA. 1968. Interstate system accident research: study II, interim report II. Public Roads 1968; 35(3): 71-75.

5. Solomon D. 1964. Accidents on main rural highways related to speed, driver and vehicle. Washington, DC: US Department of Commerce & Bureau of Public Roads.









14. - Link no longer maintained.

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