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The conventional wisdom is that established speed limits lead to an increase in safety.

Speed limits are usually set to attempt to cap road traffic speed; there are several reasons for wanting to do this. It is often done with an intention to improve road traffic safety and reduce the number of road traffic casualties from traffic collisions. In their World report on road traffic injury prevention report, the World Health Organization (WHO) identify speed control as one of various interventions likely to contribute to a reduction in road casualties.
Wikipedia: Speed Limit

I am inclined to question the conventional wisdom, especially when those proposing the restrictions have something to benefit by maintaining and enforcing restrictions (ticket revenues, enforcement disgression, etc).

To be clear, I am not talking about raising the speed limit. I think the presence of any established limit has a psychological effect on people, who may actually drive faster than is safe if a sign says that it is permissible. I am also not asking if driving a car too fast for conditions is dangerous, that seems pretty clear. I am asking if establishing and posting a legal speed limit decreases the danger on highways. I am also restricting this query to highways, since I believe that they are fundamentally different than surface and residential streets, and the data for each will be vastly different, so different conclusions may be reached.

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    Related but different question: Is driving the speed limit on a highway safer? – Brian M. Hunt Mar 14 '14 at 15:59
  • I suppose there are three cases: 1. A posted speed limit. 2. A legal speed limit which is not posted. 3. No speed limit at all. Which did you want to compare? The hard part of such a study would probably be finding a control. – Nate Eldredge Mar 15 '14 at 12:17
  • In addition to what Nate's written, it would also help if you clarify whether you're asking about speed limits that are enforced or not, as this may make a significant difference to the answer. – 410 gone Mar 15 '14 at 12:30
  • I am mostly interested in cases 1 vs 3. Case 2 is really a mix of 1 and 3 – Mauser Mar 16 '14 at 2:46
  • I feel that yes, it's possible to set a limit that's counter-productive by being too high and forcing some people to drive faster than they would drive otherwise. However, surely a very low limit cannot have this effect. So maybe clarify your question? Whether it does depends on how high it is. – RomanSt Mar 18 '14 at 12:54
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Germany is an interesting case to study this, as they have stretches of highway with and without speed limits. In a study done in the state of Brandenburg, the effect of putting a speed limit on stretches of highway has been studied by comparing the accident statistics before and after the new limit.

On two stretches the speed limit was changed from no limit to 130 km/h (~80 MPH) and the effects were quite remarkable.

Die Zahl der Unfälle (P,SS) halbierte sich von 654 U/3 Jahre auf 337 U/3 Jahre nach Einführung der Geschwindigkeitsbegrenzung (-48 %). Die Zahl der Verunglückten sank deutlich von 838 auf 362 Verunglückte in 3 Jahren (-57 %).

This says that the number of accidents in the years after the new limit decreased by 48% whereas the number of people involved in accidents decreased by 57%. (This is illustrated in the report on figure 3.1 very clearly)

Of course this has to be put into relation to the general decrease of accidents on all highways, which they did by looking at a couple of reference stretches where there was no speed limit over the whole time.

Der Rückgang zum Zeitraum 2000-2002 entsprach 50 %. Die Kontrollgruppen zeigen im Durchschnitt einen Rückgang um 23,5 % für die zeitliche Entwicklung auf, so dass die Geschwindigkeitsbegrenzung zu einer Verminderung um 26,5 % beigetragen hat. Der Unterschied der UKRa zwischen den begrenzten und unbegrenzten Strecken lag 2006 ebenfalls bei 26,5 %.

This means that the decrease (this time of the cost per distance driven linked to the accident) was 50% on the stretches with the new speed limit while it was 23.5% on the reference samples and that thus 26.5% can be linked to the new speed limit.

Overall this leads to the conclusion that in this case, a speed limit did significantly decrease the number of accidents. However I admit that this is limited to the German context, especially since it only looks at those two stretches.

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    “this has to be put into relation to the general decrease of accidents on all highways”: not exactly. It has to be compared to similar highways, not all highways. The two roads of the study may be the ones that had a high number of accidents. It would also be interesting to see whether the study mentions or not if there were other measures taken, such as warning signs indicating that the highway is dangerous, additional separators between the lanes, etc. – Arseni Mourzenko Jun 7 at 10:30
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    @ArseniMourzenko: AFAIK in Germany new speed limits are indeed put up where particular dangers are identified. This could be the road not being up to standard for the higher speed, too many and/or too severe accidents on that stretch of road, possibly also a measured or expected increase in traffic (inside towns also: new nursing home or kindergarden). What I do not know is whether there can be exceptions for the purpose of studies, nor whether it would be legal to postpone such measures in order to have matched case-control-pairs for a study when several such sites are identified. – cbeleites unhappy with SX Jun 13 at 17:55
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    Wrt. 2 German specialties should be mentioned: a) there's no general speed limit on highways, but a Richtgeschwindigkeit = recommended max of 130 km/h. The recommendation has legal consequences, e.g. in case of an accident, having been faster means being at least partially responisble for the accident. b) Trucks and trailers have a limit of 80 or 100 km/h. A speed limit on highways reduces the speed difference between the right (slow) and faster lanes, and AFAIK that difference has been identified as particularly dangerous. – cbeleites unhappy with SX Jun 13 at 18:01
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    @ArseniMourzenko They did compare to similar highways. In the study 4 reference stretches were chosen, two of which have similar or higher accident rates as the stretch in question. It is all discussed in the study with quite a lot of details and all the data shown (tables 3.1 and 3.2) – drat Jun 15 at 6:05
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    @JackAidley Link is fixed, they did account for this as the references stretches that they are compared to are of similar nature. See tables 3.1 and 3.2 for instance. – drat Jun 15 at 6:06
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Those are number from work done by the IIHS in the US, this is an interesting read about speed limits in general and not just safety:

Fatalities also went up. Deaths on rural interstates increased 25-30 percent when states began increasing speed limits from 55 to 65 mph in 1987 (Baum et al., 1991; Baum et al., 1989; Baum et al., 1990). In 1989, about two-thirds of this increase — 400 deaths — was attributed to increased speed and the rest to increased travel.

and

An IIHS study examined longer-term changes. During 1993-2017, a 5 mph increase in the maximum state speed limit was associated with an 8 percent increase in fatality rates on interstates and freeways and a 3 percent increase on other roads (Farmer, 2019). In total, there were an estimated 37,000 more traffic fatalities during these years than would have been expected if maximum speed limits in 1993 had remained in place. In 2017 alone, there were more than 1,900 additional deaths.

The quoted research The effects of higher speed limits on traffic fatalities in the United States, 1993–2017 used different variables while comparing so to isolate other influencing factors. The results are

A 5 mph increase in the maximum state speed limit was associated with an 8.5% increase in fatality rates on interstates/freeways and a 2.8% increase on other roads. In total during the 25-year study period, there were an estimated 36,760 more traffic fatalities than would have been expected if maximum speed limits had not increased—13,638 on interstates/freeways and 23,122 on other roads

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