RESEARCH - The Impact of Lowering Speed Limits in Urban Areas

MUARC LogoThe majority of all traffic collisions occur in the urban environment, where there is a more complex traffic environment and a higher predominance of road users that are more susceptible to injury and fatality in the event of an accident. A relatively straightforward and cost-effective speed management measure, involves reducing speed limits.

The relationship between vehicle speed, accident risk and accident outcome severity is well established in traffic safety literature. Research shows that reduced speed is likely to bring about a reduction in average travel speed and have a positive impact on both the number of accidents and accident outcome severity.

Other secondary benefits are also derived including: reduced fuel and vehicle operating costs, and significant reductions in vehicle emissions and noise.

The document The impact of lowered speed limits in urban and metropolitan areas is from 2008, but appropriate speed limit setting is part of Vision Zero today. This reference from the ITE, Speed Management for Safety is a guide for traffic engineers and may be of interest to the public.


Reduced limits = reduced crashes

This is not necessarily the case and I take issue with it. The author is from Monash University, Victoria State AU which has a world class traffic research facility. The assertion that reduced limits equals reduced crashes is true only in the cultural context of Victoria State"s highly successful State wide photo radar programme. That is, the limit is backed by both extensive physical and electronic enforcement. This does not apply elsewhere in the world. Practical Canadian experience with politically expedient reduced limits show virtually no change in the 85th% speeds of traffic using the road. The posted limit is almost irrelevant. There are NO Canadian Jurisdictions that have ever applied the enforcement resources to speed enforcement that the Victorians have.

As an aside I drove extensively in Melbourne in 1995 which has (or had) a universal 60kmh limit , enforced vigorously with photoradader with a 7kmh tolerance. Everyone drove smoothly on multi lane collectors at 63-64, there was no weaving, racing, tail gating, unsafe lane changes etc. Having driven extensively in 15 Nations the Victorian experience was the most orderly, peaceful, safe, stress free driving experience I could imagine. At that time the State was the safest place in the world to drive. Regrettably Canadian politicians and police agencies have never set the priorities in the way the Victorians did from 1989 on. The author is accurate about reduced speed limits, but only in the context and culture of his State.

David M. Greenhalgh

Interesting comment about prevailing traffic speed

I note that the "Victorians" drove at 63-64 km/h - in excess of the speed limit.  That indicates to me that they were pretty much driving at what they figured they could get away with, which I believe is quite typical where speed limits are set below the natural 85th percentile level. 

I find it interesting to drive in states like Montana, where there seems to be a significant number of drivers who drive slower than the posted 70 mph (113 km/h) daytime limit on two lane highways.  Yes, this actually happens when speed limits reflect actual 85th percentile speeds. 

I prefer the 85th percentile approach as the most reasonable and democratic, regardless of relative safety.  All movement involves risk, there's no magic to speed limits, and it's most appropriate in a democracy to let the majority decide what level of risk its comfortable with.  I would suggest that if the speed limit is treated by most drivers as a minimum, the limit is probably set too low.

FWIW, I've also lived in a small BC town where 85th percentile speeds (based on local speed surveys) seemed to be well under 40 km/h on local streets, despite the 50 km/h limit.  There were very few speeders, despite resident perceptions!  I think the short blocks, poor sightlines at many intersections, and the slow pace of life in general all contribute to this situation. 

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