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. http://en.wikipedia.org/wiki/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
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    According to our Privileges section, you should only use comments to request clarification from the author or leave constructive criticism that guides the author in improving this post. Please review the When shouldn't I comment? section and act appropriately in the future. – Sklivvz Mar 15 '14 at 11:16
  • 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. – EnergyNumbers 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

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 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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