Effectiveness
A 2003 review of speed limits for British Columbia (Canada) identified a number of studies into the effects of reductions in speed limits and the observed changes in speeding, fatalities, injuries and property damage which followed:
| Country | Speed limit reduction | Reported change |
|---|---|---|
| Australia | 110 km/h to 100 km/h | Injury crashes declined by 19% |
| Australia | 5–20 km/h decreases | No significant change |
| Denmark | 60 km/h to 50 km/h | Fatal crashes declined by 24% Injury crashes declined by 9% |
| Germany | 60 km/h to 50 km/h | Crashes declined by 20% |
| Sweden | 110 km/h to 90 km/h | Speeds declined by 14 km/h Fatal crashes declined by 21% |
| Switzerland | 130 km/h to 120 km/h | Speeds declined by 5 km/h Fatal crashes declined by 12% |
| UK | 60 mph to 40 mph | Speeds declined by 4 mph Crashes declined by 14% |
| USA (22 states) | 5 mph to 15 mph (8 km/h to 24 km/h) decreases | No significant changes |
| Country | Speed limit increase | Reported change |
|---|---|---|
| Australia | 100 km/h to 110 km/h | Injury crashes increased by 25% |
| Australia (Victoria) | 5–20 km/h increases | Crashes increased overall by 8%, 35% decline in zones raised from 60 km/h to 80 km/h |
| USA | 55 mph to 65 mph (89 km/h to 105 km/h) | Fatal crashes increased by 21% |
| USA | 55 mph to 65 mph (89 km/h to 105 km/h) | Fatal crashes increased by 22% Speeding increased by 48% |
| USA (40 states) | 55 mph to 65 mph (89 km/h to 105 km/h) | Fatalities increased by 15% Decrease or no effect in 12 States |
| USA (Iowa) | 55 mph to 65 mph (89 km/h to 105 km/h) | Fatal crashes increased by 36% |
| USA (Michigan) | 55 mph to 65 mph (89 km/h to 105 km/h) | Fatal and injury crashes increased significantly on rural freeways |
| USA (Michigan) | Various | No significant changes |
| USA (Ohio) | 55 mph to 65 mph (89 km/h to 105 km/h) | Injury and property damage increased but not fatal crashes. |
| USA (40 states) | 55 mph to 65 mph (89 km/h to 105 km/h) | Statewide fatality rates decreased 3-5% (Significant in 14 of 40 states) |
| USA (22 states) | 5 mph to 15 mph (8 km/h to 24 km/h) increase | No significant changes |
Annual surveys of speed on South Dakota Interstate roads show that from 2000 to 2011, the average speed rose from 71 to 74 mph; South Dakota increased its maximum speed limit from 65 to 75 mph (120 km/h) in 1996.
The Synthesis of Safety Research Related to Speed and Speed Limits report sponsored by the Federal Highway Administration published in 1998 found that changing speed limits on low and moderate speed roads appeared to have no significant effect on traffic speed or the number of crashes, whilst on high-speed roads such as freeways, increased speed limits generally resulted in higher traffic speeds and more crashes. It is also stated that there is limited evidence to suggest that speed limits have a positive effect on a system wide basis.
Research in 1998 showed that the reduction of some 30 mph (48 km/h) United Kingdom speed limits to 20 mph (32 km/h) had achieved only a 1 mph drop in speeds and no discernible reduction in accidents; '20 mph speed limit zones' which use self enforcing traffic calming achieved average speed reductions of 10 mph, child pedestrian accidents were reduced by 70% and child cyclist accidents by 48%.
Zones where speeds are set at 30 kph (or 20 mph) are gaining popularity as they are found to be effective at reducing crashes and increasing community cohesion.
Studies undertaken in conjunction with Australia's move from 60 km/h (37 mph) speed limits to 50 km/h (31 mph) in built-up areas and found that the measure was effective in reducing speed and also the frequency and severity of crashes.
A study of the impact of the replacement of 60 km/h (37 mph) with 50 km/h (31 mph) speed limits in New South Wales, Australia showed only a 0.5 km/h drop in urban areas and a 0.7 km/h drop in rural areas. The report noted that widespread community compliance would require a combination of strategies including traffic calming treatments.
Information campaigns are also used by authorities to support speed limits, for example the Speeding. No one thinks big of you. campaign in Australia 2007.
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