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I'd have thought this would have been asked already but I didn't see it. Is driving the speed limit safer on a highway? By "highway" I mean high-traffic areas, such as 4-lane divided thoroughfares.

Having commuted for some time, I've noticed that people who clearly drive at excessive speeds seem to be dangerous. However, more often than not the danger I see is from people who are driving slower than the prevailing rate (where the prevailing rate is generally more than the speed limit). This seems more dangerous because it disrupts the flow of traffic, irritates drivers and people behind the slow driver end up tailgating.

Is there any evidence to suggest that:

  1. speed limits on highways improve the safety of people driving on those highways?
  2. people driving the speed limit and those around them are safer than those travelling at the prevailing rate of traffic - even where that prevailing rate is faster than the speed limit?

EDIT It is noteworthy that there has been news recently that Google's self-driving car is programmed to go faster than the speed limit, apparently because it can be safer in some scenarios.

EDIT In line with this is a comment from the video Why you shouldn’t drive slowly in the left lane by Vox, which states “Research has shown that the strongest predictor of an accident is variance from the average”. Which seems obvious (and hence this question), but I would like to know what the paper is and whether it's been debunked.

Oddthinking
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Brian M. Hunt
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    Aren't the main contributing factors in serious accidents 1) excess speed, 2) alcohol, with 3) not wearing seat belts, 4) being young and invincible, and 5) distraction by cell phone use, coming up behind? Yeah, people driving the limit may irritate younger drivers, but in a sense they are doing them a favor. – Mike Dunlavey Jun 02 '11 at 21:23
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    @Mike: More speed is more energy, and more energy is more problems if an accident happens. However, if everybody's going the same speed, accidents are less likely, and so there may be fewer deaths with everybody going about 70 mph as opposed to some at 70 and some at 55. I don't know how to test that. – David Thornley Jun 03 '11 at 02:48
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    You are asking two different questions here, and although they're related I'm not sure they're close enough to make one good question. – David Thornley Jun 03 '11 at 02:50
  • the effect of enforcement of a speed limit is also problematic compared to, say, a speed advisory. If there is a speed limit of 55, and because a policeman notices a 'speeder' driving 80, the speeder decides to drive 130 to try to evade, there is an issue. It is not immediately obvious whether enforcement makes highways safer or not. – Paul Jun 03 '11 at 09:17
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    @Mike I was under the impression speed differentials were one of the major contributing factors. If everybody is driving 70 it will probably be fine, but it you have somebody sitting in the middle lane at 30 that is unexpected so a group of cars coming up behing that are doing the speed limit need to take avoiding action, if they fail and accident is caused – Ardesco Jun 03 '11 at 13:49
  • Even on german Autobahns, you have differnt speeds. There is always a speed limit for trucks of 80 km/h, then there are busses, afaik, which are allowed to drive 100 km/h. If there is a route section without general limit, the drivers choose a speed depending on their personal favor, maybe influenced by their cars engine. But if there is a general limit of - let's say 120 km/h, you'll find a lot of drivers which exceed the limit slightly, because they know of the police' tolerance, and drive 130, but there is never a complete synchronized prevailing rate. – user unknown Jun 03 '11 at 17:55
  • But since the prevailing rate doesn't differ too much from the speed limit, therer will rarely be a new danger, induced by slowing down a few km/h. – user unknown Jun 03 '11 at 17:57
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    it's not the speed that's dangerous, it's the difference in speed between vehicles. If everyone drove 100mph, there's no trouble. But if one guy drives 100mph and comes up on a plug of cars driving 40, there's a big problem unless he sees them well in advance and can slow down. And with trucks being limited in how fast they can go, there's an upper limit to what you can allow passenger cars to safely drive unless you have 1) a lower speed limit for everyone and 2) ban trucks from ever leaving the slow lane. – jwenting Jun 06 '11 at 08:16
  • in Germany that works in areas where there's no speed limit. Most accidents happen at relatively low speeds anyway, like tailgating during traffic jams and on turnoff and entry ramps and are caused not by excessive speed but by people not keeping enough distance and simply not watching what's happening around them before changing lanes. – jwenting Jun 06 '11 at 08:17
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    Driver's license qualification in Germany is also a lot stricter than it is in the US, so in addition to the above arguments, this really *is* an apples-to-oranges comparison. It would be good to remove this portion of your question. – Ernie Jun 10 '11 at 16:36
  • @Ernie: Agreed; done. – Brian M. Hunt Jun 10 '11 at 18:44
  • @Ardesco: if the one going 30 does startle (is wake up the term?) so a hectic braking results, then the distance and or speed of the one behind was not appropriate. Likewise, if someone behind the 70-going-one needs to brake hard. After all, there could always be an accident after the curve, a deer on the road or whatever. – cbeleites unhappy with SX Oct 14 '12 at 22:32
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    Despite what any poll or survey might say, the number one cause of accidents are people not paying attention, or simply being a lazy driver. Yeah, I'm looking at you ***Mr. I change lanes without using my turn signal because it's so hard to reach two inches down to activate it.*** Afterall, I should be reading your mind and know that you want to change lanes, right? – crush Mar 14 '14 at 16:11
  • "it disrupts the flow of traffic, irritates drivers, and people ... end up tailgating." You could say this about red lights. –  Nov 02 '18 at 16:19

4 Answers4

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Higher average speed will lead to more accidents. This is because braking distance doesn’t just double from 100 km/h to 200 km/h. In fact, it quadruples. It’s nonlinear, but people think it’s linear. Insufficient following distance is the second-biggest cause of accidents (the biggest is excessive speed). Translate this link.

You might have thought that big traffic jams are mainly caused by accidents. But in fact, it’s the difference in speed that will on higher speed increase non-linearly probability of abrupt braking, insufficient car-to-car distances and longer braking distance of cars. Modern traffic systems try to decrease the human factor as much as possible in order to reduce braking distance. Then you can increase speeds. This is because several cars will behave like a railway train with fixed distances between train carriages.

The second point you mention to adapt to avg. speed (> speed limit) to drive safer is kind of misleading and mirroring this linear thinking (esp. for very old drivers with reduced reflexes), as people will tend to choose non-linearly too small distances to the car in front with increasing speed. Trucks in the right-hand (slow) lane are very unlikely to cause traffic jams. This is because they behave mostly like the "road train" that was described in the hyperlink in the previous paragraph.

We do a lot of traffic research here in Germany. This is because some of our speed-limit signs are computer-controllable, depending on traffic density, average speed, and traffic-jam messages from drivers. These limits get adapted temporally on highly frequented highways, but only downward from the maximum speed limit. Also you probably know of new car features like "Autonomous cruise control system", that will use computer control to control the speed and distance to cars in front to you.

So of course if the speed limit is 100 km/h, one shouldn’t drive 50 km/h in the rightmost lane. Small speed-limited two-wheeled motor vehicles (max. 50 km/h) are forbidden on German highways. As far as I know, in Europe the highway speed limit of around 120 km/h and an advisory speed limit around 80 km/h is the best compromise of fast traffic and human factor. In Germany we have, in some areas, no general speed limit, since we are car fanatics and have a strong lobby :) However, most European countries have a general speed limit of around 120 km/h.

unforgettableidSupportsMonica
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Werner Schmitt
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    The breaking distance is just one of many factors. The difference in speed is another. Although it isn't specifically mentioned, I do not thing Autobahn is a reasonable comparison as the Autobahn generally has different speed limit per lane, with some having "no limit". Thus, the counter effect mentioned by the original poster of maintaining the prevailing speed (other than the speed limit) would not apply. – Tormod Jun 03 '11 at 16:30
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    Being in police work for many years, I have been interested in the various aspects of traffic safety. Read a fascinating book last year called "Traffic, or why we drive the way we do." Dealt as much with the psychology of driving as actual safety measures. A lot of cherished notions turn out not to be well-tested or supported. IMO, speed considered alone is rarely a factor. When you throw in inattention, tailgating, failing to adjust for conditions... It's perfectly safe to drive at 200 mph bumper-to-bumper, NASCAR guys do it all the time. Till something goes wrong. – M. Werner Jun 10 '11 at 19:55
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    @Tormod No, the autobahn doesn't generally have different speed limits per lane. – Ruben Jun 10 '11 at 21:00
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    This answer could do with some more references to support some of the claims. – Oddthinking Apr 16 '12 at 11:28
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    Certainly in the UK the statistics actually do not support the claim that higher average speeds lead to more accidents. There is a strong correlation with the damage done in the event of an accident, but the major causes are lack of concentration, lack of training, lack of experience and tiredness. see http://www.safespeed.org.uk/ – Rory Alsop Aug 31 '12 at 09:50
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    @Rory Alsop **lack of concentration, lack of training, lack of experience and tiredness** all of these are worse at higher speeds. – Stefan Aug 31 '12 at 12:49
  • @Stefan lack of training, lack of experience should translate to going slower (for safety reasons). Also tailgaiting of course is related to speed. You're not to close, just too fast for the distance to the one before you. Lack of concentation and tiredness should translate to much slower = 0 speed... – cbeleites unhappy with SX Oct 14 '12 at 22:29
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    @cbeleites It should translate, but it doesn't. Why? Drivers who have a lack of training and experience don't understand the dangers they are causing. – crush Mar 14 '14 at 16:22
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    *"most European countries have a general speed limit of around 120 km/h."* Actually 130 seems to be more common. – vartec Jun 20 '14 at 08:39
  • @vartec: Some are higher, some lower, but in general you're quite right: http://upload.wikimedia.org/wikipedia/commons/thumb/e/e2/European_Speed_Limits.svg/600px-European_Speed_Limits.svg.png – David Mulder Jun 21 '14 at 12:01
  • But increased speed logically leads to increased concentration. The question is, is a lack of concentration worse than too much speed? – Jonathon Sep 08 '14 at 16:21
  • I actually compiled BTS data on this many years ago and it was not speed that kills. Faster did not equal more accidents (on highways). Actually highway speeds in the USA have increased since the 70s but accidents per highway mile driven have decreased. Driving too close to the car in front was found to be the single biggest cause of accidents. Basically you can all go 100 mph if you keep the correct spacing. If I remember though your chance of a fatal accident did increase with speed just not your chance of an accident. check out the USA BTS for mor info. – If you do not know- just GIS Aug 27 '16 at 22:52
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This is a near-miss answer: it addresses only 60 km/h roads, not highways. Nonetheless, it addresses some of the attitudes to driving described in the question, so it may shed some light on the answer:

For 60 km/h speed-limits:

  • Driving faster than the speed limit is associated with a huge increase in the chance of being involved in an accident involving a casualty.

  • Driving slightly slower than the speed limit doesn't make much difference.

  • Driving very slowly (e.g. 40 km/h in a 60 km/h zone) is associated with a moderate increase in the chance of being involved in an accident involved a casualty.

This is illustrated in Table 4.3 of a 1997 report from the NHMRC Road Accident Research Unit of The University of Adelaide: Kloeden CN, McLean AJ, Moore VM, Ponte G, Travelling Speed and the Risk of Crash Involvement, Volume 1 - Findings.

Their headline finding:

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.

Graph of relative risk

Correlation is not causality. Correlation is not causality. Correlation is not causality.

Oddthinking
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    But the stats leave out key issues of some relevance. Are they based on the absence of other road users? Would a similar pattern exist if the road were designed for 120km/hr travel? I imagine that going very fast on a road with a 60km/hr limit where most users obey the limit is very unsafe but doing the same on an uncrowded motorway might not be. – matt_black Apr 16 '12 at 23:16
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    Yes, the roads would be safer if there were no other road users, the roads were designed for faster travel, people were trained to drive faster, all roads were straight, and people were made of titanium. :-) I can't see the point of your hypotheticals. I will grant (and do in the first sentence) that stats for 60 km/h roads are not necessarily 100% applicable for 100 km/h roads - I don't think they can be totally dismissed though. – Oddthinking Apr 17 '12 at 00:59
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    One thing to point out is that these stats point to risk of casualty (ie death) and not just risk of injury or even just risk of a crash. Of course there are going to be more deaths 70 or 80 km/h then at 60, because the car is travelling at a much higher speed, and is carry a lot more energy (E = 1/2 mv^2). Also cars are pretty good at protecting us at low speeds but they aren't designed to protect you as well in highspeed crashes. Crash tests in the US are only done at 35 mph (56 km/h) because the chances of surviving a high speed crash are very low. – Kibbee Sep 01 '12 at 00:37
  • What about selection-bias? – user1873 Jun 19 '14 at 22:33
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    @user1873: I am trying to understand what you mean. If you mean only poor drivers elect to speed (or drive significantly lower than the speed limit), you have a fair point. This is one of the reasons I have reminded people I haven't shown causation. – Oddthinking Jun 20 '14 at 01:54
  • @Kibbee - you seem to be making a great argument for going slowly, even when the "Flow of traffic" is faster. I'd take a 100% change to be in an accident where no injuries occur over a 20% chance of being in an accident where I die. – Selkie Nov 02 '18 at 17:50
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Several of the other answers here talk about speed, energy and braking distance. But that's not what the question asked.

The question asked whether it is safer to drive at the speed limit, and makes reference to other cars driving both faster, and slower than this figure.

The answer to this is that there's no objective answer. Speed limits are a legal construct, and have no intrinsic relation to physics or engineering.

By far the biggest factors in road safety are:

  • Paying due care and attention to those around you, and being alert for potential hazards, while taking mitigating action to avoid them, and;
  • Driving at an appropriate speed for the conditions at the time.

The appropriate speed depends on many factors, for example the density of the traffic; whether that section of road is straight and clear (or narrow and winding); presence of junctions; presence of parked cars; weather (snow, ice, fog, rain, or a clear dry day); driver experience; driver reaction times; and braking ability of the car.

This is not an exhaustive list. However it illustrates the complex factors which will come into any conceivable equation when calculating "What is the safest speed". Clearly, this is a question which can not be answered objectively.

With regards legal speed limits, you tend to find the same speed limit applied to the same road - regardless of whether one section is straight and clear, but another section has many junctions, or is a common place for cars to park at the side. Furthermore, most speed limits don't change on a daily basis according to the weather. This is why on a clear, dry day many legal limits feel absurdly low - whereas in snow and ice, on the same stretch of road the legal limit may be dangerously fast.

Part 2 of the questions asks whether it's safer to drive at the legal speed limit even when the prevailing speed of traffic is faster. Again, the answer is: "it depends". If the prevailing speed of the traffic is excessive, and the traffic is so dense that stopping distances are compromised (i.e it's actually an unsafe speed), then it would indeed be safer to reduce your speed to increase the distance between your car and the one in front (even at the expense of the annoyance of drivers behind you.

On the other hand, if the density of traffic is relatively light and there is plenty of stopping distance, then it's safer to go with the flow. The reason for this is because if you're driving significantly slower than the rest of the vehicles around you, you're causing them to swerve around you; change lane; and otherwise alter their behaviour by making them take additional manœuvres. At high speed, this can create more inherent danger than simply factoring in the energy calculation.

To recap: overall, higher speeds involve more energy, and therefore a reduction in the overall energy on the roads means that in the event of an accident, there is less chance of damage and loss of life. However this is unrelated to the "legal speed limit"; and there are many other important factors to road safety. Drivers who spend their time with their eyes fixed on the speedometer are generally paying less attention to the road around them.

Chris Melville
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    Despite being a legal construct, speed limits *do* have a relationship to physics and engineering. Thankfully, policy is sometimes informed by science. – De Novo Nov 02 '18 at 16:51
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    Do Novo - care to explain your logic? Care to explain why my narrow residential road, with loads of parked cars, has a legal limit of 30mph; while the nearby main road (wide and open) also has exactly the same legal limit of 30mph? There's no physics-related reason for that. It's all just about a nice round number being decided upon by the government and slapped onto all roads in the area, regardless of their features. It's a political construct: not a physical one. – Chris Melville Nov 02 '18 at 21:40
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    No need to invoke logic. It just happens to be done that way. Legislative bodies consult traffic engineers, who provide recommendations based on physics and engineering principles. Policy, meet science :) – De Novo Nov 02 '18 at 21:52
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    "No need to invoke logic" - then no point in continuing this discussion. – Chris Melville Nov 02 '18 at 21:54
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    [Welcome to Skeptics!](http://meta.skeptics.stackexchange.com/questions/1505/welcome-to-new-users) This is an answer based purely on a theoretical model. We expect answers to be based on empirical evidence rather than speculative predictions. Please edit it to add references to empirical data. – Oddthinking Nov 03 '18 at 00:12
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    "Speed limits are a legal construct, and have no intrinsic relation to physics or engineering." This is an empty argument. Which side of the road you drive on is purely a legal construct, and yet driving on the wrong side of a highway is empirically much more dangerous. – Oddthinking Nov 03 '18 at 00:13
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    Re: "no need to invoke logic", the point here is that you don't have to rely on an argument proceeding from assumptions and using speculation about what is or isn't likely to follow from those assumptions. It just happens to be the case that speed limit decisions are informed by science, at least in the US, where I'm familiar with the process. – De Novo Nov 03 '18 at 02:41
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  1. That depends on the specific highway. For example, in British Columbia, it seems as if increasing the speed limit actually causes accidents to go down (source - there's a video near the end of the page that explains why this is so). You have probably heard about the German Autobahn, where in some stretches there's no posted limit, and yet it's famously safe to drive on (in 2011, there were 5.6 fatalities in German roads per billion vehicle-km - source).
  2. No, people are safer when everyone is driving at the same speed, going in the same direction. The first link above has several sources to back up this claim.
LmnICE
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    Welcome to Skeptics. 1: Changing the speed limit is a separate question. 2. Could you quote the part that makes this claim? Even better, chase it up and quote the original sources. – Oddthinking Mar 21 '14 at 02:32