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The UK government has recently started a consultation on increasing the speed limits on motorways (roughly the equivalent of highways in the USA) in England.

So it seems appropriate to pose the question about whether this is a good idea. The early debate has highlighted the typical concerns of safety and worse fuel economy. So the usual suspects have already chimed into the debate with these sorts of arguments:

But the Green Party said the plan would increase carbon emissions.

Stephen Joseph, from the Campaign for Better Transport, said it would lead to more accidents and casualties. ...

Road safety charity Brake's chief executive Mary Williams said: "This is a selfish move that will achieve nothing other than carnage and is pandering to an uninformed few.

"What is far more legitimate is the grief of families bereaved on Britain's motorways in horrendous pile-ups at high speed, and the rights of all UK citizens to have slower, not faster, speeds on roads to enable drivers to avoid collisions." ...

Green Party spokesperson Jenny Jones said: "This is a mad idea just at the time we should be worrying about fuel economy and emissions. Putting the speed limit up will be worse for both. So much for the 'greenest government ever'."

The department of transport counter some of these arguments with:

advances in technology have made cars much safer, contributing to a drop of more than 75% in the number of people killed on British roads since the 70mph limit was introduced.

So my question is are any of the concerns significant in the light of real evidence? I deliberately use the word significant because small effects don't seem to me to make a compelling argument against the policy. Another factor that good answers should address is the relationship between the policy and actual behaviour on the roads (only about half of all drivers in the UK stick to the current limit). I've also deliberately kept the question narrowly focussed on motorways which are by far the safest roads in most countries. What happens on other road types is not my concern.

  • "What happens on other road types is not my concern." Unless of course you don't manage to teleport yourself from your driveway to the motorway and actually have to drive there. On other road types ;-) – Zano Oct 5 '11 at 11:40
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    Could you please clarify what you mean by "significant"? I ask because I think this is going to be a critical point. For some people, a 10% rise in deaths per annum is not going to be significant. For others, a single extra death is significant - to them arguing that there has been a 75% drop is not a justification for further speed increases. If significant is left undefined, this is not a science question but an ethics question. – Oddthinking Oct 5 '11 at 12:07
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    @zano I'm not concerned about other road types for two reasons: the speed limit isn't going to change on them and Motorways are already almost an order of magnitude safer than other roads despite their faster speeds. – matt_black Oct 5 '11 at 14:45
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    @Oddthinking My intention with significant was twofold. One is whether any difference is likely to be even noticeable statistically; the other is whether the difference is large compared to other safety issues on roads (subjective, I admit, but I want people to quantify the impact and recognise the tradeoffs, rather than argue that there is bound to be some small effect so it is a bad idea to change the limits). – matt_black Oct 5 '11 at 14:54
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    As everyone, including the government now, agrees, speed on motorways is not what causes accidents. Inattention, inexperience and tiredness are the problem. What raising the speed limit will do is move a lot of people from breaking the law to being within it. The other proposed limit change is likely to have greater impact-reducing speed limits in town. – Rory Alsop Oct 5 '11 at 16:17
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There are several separate concepts to be addressed: fuel-efficiency, accident rate, fatality rate and experience in other countries.


Does vehicle speed affect the fuel-efficiency?

Fuel efficiency is measured in mpg or Litres/100 km - i.e. factoring out the speed travelled, and just looking at the distance you can get per volume of fuel. I am assuming that fuel-efficiency is a reasonable proxy for g(CO2). This ignores tire-wear, vehicle and road damage, change in transport consumption due to faster times, etc.

Fuel efficiency is generally reduced for high speeds e.g. higher than 55 mph (88 km/h). [Ref: Consumer Reports test]

Note: The Oak Ridge National Laboratory claims [Figure 4.2] that some modern cars have better fuel efficiency at 65 mph than 45 mph, but that (with occasional exceptions) 55 mph is still better - the slope points downwards for higher speeds. Wikipedia has a nice graph.

It seems reasonable to conclude that if the typical speed of cars is increased above 55 mph, average fuel efficiencies will drop.


Does vehicle speed affect the risk of accidents?

According to the World Health Organisation (WHO), YES.

WHO's World Report on Road Traffic Injury Prevention states that:

There is a large amount of evidence of a significant relationship between mean speed and crash risk.

[...]

Empirical evidence from speed studies in various countries has shown that an increase of 1 km/h in mean traffic speed typically results in a 3% increase in the incidence of injury crashes (or an increase of 4–5% for fatal crashes), and a decrease of 1 km/h in mean traffic speed will result in a 3% decrease in the incidence of injury crashes (or a decrease of 4–5% for fatal crashes).

[...]

A meta-analysis of 36 studies on speed limit changes showed, at levels above 50 km/h, a decrease of 2% in the number of crashes for every 1 km/h reduction in the average speed.

These claims are all referenced to the appropriate studies in the report.

For comparison, they also map excess speed to the equivalent Blood Alcohol Level equivalent.


Does vehicle speed affect the risk of accidents involving serious injuries and fatalities?

According to the World Health Organisation (WHO), YES.

Again, the WHO's World Report on Road Traffic Injury Prevention states that:

The probability of a crash involving an injury is proportional to the square of the speed. The probability of a serious crash is proportional to the cube of the speed. The probability of a fatal crash is related to the fourth power of the speed.

[...]

For car occupants, the severity of crash injury depends on the change of speed during the impact, usually denoted as Δv. As Δv increases from about 20 km/h to 100 km/h, the probability of fatal injuries increases from close to zero to almost 100%.

They go on to discuss pedestrian survival rates when hit at high-speeds. Pedestrian accidents are, perhaps, unlikely on motorways, but the figures are quite appalling.


What is the German experience?

Germany, famously, has autobahns with areas where speed-limits are not enforced on some vehicles.

Some people have claimed that Germany does not have an increased accident-rate in areas with no blanket speed-limit. For example, Wikipedia's page on autobahns states:

The overall road traffic safety of German autobahns is comparable to and in some cases better than that of other European highways.

[...]

Moreover, international accident statistics demonstrate that limited access grade separated roads such as Autobahns and motorways have much greater road traffic safety regardless of speed limit, suggesting that high speed alone isn't a deciding factor.

I haven't found such evidence, yet. I hope someone can add to this answer.

The International Transport Forum's International Traffic Safety Data and Analysis Group (IRTAD?) has a database of figures by country. Looking at the IRTAD figures, Germany has a relatively low death rate by population and by vehicle km travelled. (Compare it to neighbours with similar terrain and climate.)

However, these figures don't separate out autobahn travel from other roads, and the unlimited sections of the Autobahn only account for a small percentage of the total travel. (Wikipedia claims "only two percent of the traffic in Germany runs on unlimited sections" without an explicit reference to support that.)

UPDATE: A fact sheet (non-peer-reviewed?) from the European Transport Safety Council (ETSC) sheds some light on this. Speed Fact Sheet #1, Feb 2008.

It points out that:

One argument used in favour of the status quo is that a similar proportion of deaths on the motorways in Germany occur on sections without speed limits as on sections with limits. According to the German Statistics Agency, of the 645 road deaths that occurred on motorways in 2006, 441 or 67% occurred on motorway sections without limits. This is consistent with the figures from the previous year, 2005, when 662 deaths occurred on motorways, 462 (70%) of them on unlimited sections (fig. 3):

But this similarity of percentages takes no account of traffic volumes on the different sections. Traffic volumes on sections without speed limits are likely to differ appreciably on average from those on sections with speed limits. This should be investigated to cast more light on the numbers of deaths on the two kinds of section.

Some graphs are provided that show, with limited statistical rigour, that a disproportionately high number of lethal accidents occur, per km of road, in unlimited areas. (To my eye, they seem to undermine the original claim of similar death rates for speed-limited and unlimited areas.)


Conclusion

I think it is clear from the evidence that, all else being equal, high-speeds significantly decrease fuel-efficiency, significantly increase the risk of accidents, and dramatically increase the risk of fatal accidents.

Whether this environment cost and cost in lives is worth it is not a scientific question. It is an environmental, economic, ethical and political one. If you wish to be allowed to speed anyway, fine, but don't expect the science to say it is without any cost.

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    I'm not yet convinced by your conclusion, but I like the mix of evidence you have found. – matt_black Oct 5 '11 at 21:48
  • We got to the same point through different paths :) – Sklivvz Oct 6 '11 at 7:33
  • Hat tip to @Jivlain for updating both this answer and Wikipedia to fix a broken reference. Thanks. – Oddthinking Oct 6 '11 at 15:22
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    Just so you know, cars are built to optimize their fuel efficiency for a certain speed. At the moment, especially in the United States, they are built for the highest fuel-efficiency at approximately 55mph (highway speed), but in Germany for example, they will produce cars that have their highest fuel efficiency at a much higher speed. – Asaf Jan 10 '12 at 14:34
  • Have you a reference for that, Asaf? – Oddthinking Jan 10 '12 at 16:08
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Safety

I am going to answer with basic physics here:

K = ½ m × v2

A 1200 Kg car moving at 30 m/s (~70 mph), has a kinetic energy of

K = ½ 1200 Kg × (30 m/s)2 ≅ 500 KJ

A 1200 Kg car moving at 35 m/s (~80 mph), has a kinetic energy of

K = ½ 1200 Kg × (35 m/s)2 ≅ 750 KJ

Therefore, increasing the limit from 70 mph to 80 mph (by ~15%) will increase the energy carried by cars going at the limit by 50%.

50% more energy is a significant difference, if you have an accident.

Note, this also means that the driver has 50% more energy.

When you have an accident, the kinetic energy of the car is used to do work--in practice to deform the vehicle, etc.

Also, the kinetic energy of the driver is used to do work--in practice deform the body of the driver, breaking bones, organs... ouch!

No matter how advanced the security systems are, the government is still increasing the potential for harm by 50% by increasing the speed limit to 80 mph.

In other words: if the newer cars are safer than old models, they are even safer at 70 than 80!

If you prefer to compare videos:

70 mph

80 mph

Environmental impact

CO2 emissions (per Km) are a function of surface area and weight of the car, which would not change.

There would be a major change, if this law increased the demand for more powerful cars, which are less efficient.

Finally, cars going faster experience more drag, in quadratic proportion to velocity. If we measure emissions per km, there is still a linear increase left. In other words, increasing speed by 15% will result in 15% more emissions, all other factors being equal.

References:

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    The physics is clear, but does that suggest I should never travel on a train as they hit 130mph even on UK commuter lines? Safety can never be the only consideration or we would never let anyone travel faster than walking pace and people with red flags walking in front of vehicles would still be compulsory. So is this proposed tradeoff significant or should we focus on something else? – matt_black Oct 1 '11 at 23:16
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    @Sklivvz - that analysis is factually correct yet somewhat unhelpful to the complete picture (as you yourself said, ceterus parabus). In order for it to be meaningful safety-wise, you need to show that 3 things are true: (a) A large part of the accidents produce outcomes where increasing the energy by 50% will qualitatively impact the outcome (as simple example: if ~100% of accidents at 70mph are fatal, then 80mph doesn't matter). (b) That 15% decrease of travel time would not offset the increased risk from more energy. (c) Ditto decreased risk of 1/2 the people already driving 80 and 1/2 70 – user5341 Oct 2 '11 at 15:55
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    @Sklivvz - basically, you merely argued for increased risk from one factor (and not quantified that risk visavi actual car safety design in 2011 - they might very well already have 50% extra margins on older cars safety wise, or just 5% ), without nearly accounting for comprehensive multi-factor picture, yet your answer does NOT in any way stress that fact. – user5341 Oct 2 '11 at 16:01
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    @Sklivvs - yet another important factor is that (possibly) a lot of accidents involve drivers who ALREADY go 80mph (see OP's explicit clarification at the end) and therefore increase of legal limit would have negligible effect on accident rate. Again, I'm not saying it won't - merely that it's a VERY important fact to consider which your original research answer doesn't account for. – user5341 Oct 2 '11 at 16:03
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    @Sklivvz: that's not how it works. See for example this graph en.wikipedia.org/wiki/File:Fuel_economy_vs_speed_1997.png using data from table 4.27 here cta.ornl.gov/data/tedb30/Edition30_Chapter04.pdf . All the tested vehicles have their mpg drop off past 60mph. This is roughly because energy per mile lost to air resistance goes like the square of the speed (power lost goes like the cube). (Drag is not the only factor at lower speeds, which explains why there is a peak in mpg somewhere around 40-60mph). – Lev Bishop Oct 5 '11 at 16:20
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Yes, faster speeds will increase accidents, but not by much if they are restricted to motorways

A BMJ editorial has now joined this debate with arguments similar to those described in the first two answers given above. It concludes:

Any potential economic benefit is likely to be outweighed by the adverse effects on health

The key evidence it uses is based on natural experiments when other countries altered their speed limits. The article argues:

The example that is arguably the most comparable to the UK proposal was the increase in speed limits in many US states after 1995 when the national maximum speed limit, introduced in response to the 1974 oil crisis, was repealed—limits on interstate highways and freeways were increased, typically from 65 mph to 70-75 mph or from 55 mph to 60-65 mph. This was associated with a 16.6% increase in deaths.

And that sounds like a compelling public health case. But you should always be suspicious about percentages as it is easy to exaggerate when using them.

The piece of information not given is the number of deaths expected in the context of all road deaths. When I posed the question I asked whether there would be a significant increase and I expected the context for significant to be all road deaths. 16.6% sounds significant. But motorway deaths don't account for many road deaths in the UK (it is likely that something similar applies in most countries, though UK motorways are particularly safe). Conveniently the BBC recently produced an interactive visualisation of UK road deaths with a variety of useful classifications (see the interactive pages here).

The key statistic is this: out of more than 3,000 annual deaths they report fewer than 150 on motorways. That is less than 5% of the total. Even if these increase by the amount seen in the USA on increased speeds, the total extra deaths would be about 30 a year (<1%) or so small as to be in the noise from natural variation from year to year. Neither the editorial or the previous answers provided this context.

I draw from this two conclusions: what happens on motorways isn't significant in the context of road safety as a whole; if we are going to do anything the focus should be elsewhere.

We could make roads perfectly safe by making a man walk in front of all cars waving a warning flag. But we accept some degree of risk because of the major benefits of driving. Changing the speed on motorways is not a significant issue in this context.

  • Where do most of the accident deaths happen, then? BBC’s “x road” and “unclassified” are not very helpful. In particular, why do so many more accidents happen on A roads than on motorways? – Konrad Rudolph Apr 19 '12 at 23:18
  • @KonradRudolph It is easy to explain: A-roads are much less "regular" than motorways in their design. Motorways are built so visibility is always good, curves are gentle and regular, exits and joins are safe and highly standardized and signage is very good and very consistent. Plus driver behaviour is relatively constant (eg range of relative speed is not large). Other roads have none of this and dangerous features like blind junctions and sharp curves are far more common. Plus, drivers behave less predictably. Plus, there is a much higher total traffic volume. – matt_black Apr 20 '12 at 12:03
  • Okay. Next question then: was the speed limit on A roads also raised in the same time frame as for motorways? – Konrad Rudolph Apr 20 '12 at 12:14
  • @KonradRudolph It is currently a proposal only (in the UK) and only applies to motorways. – matt_black Apr 20 '12 at 14:40
  • Ah, I mixed that up with the US speed increase that you cited. – Konrad Rudolph Apr 20 '12 at 15:10

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