Aug 10, 2012
Variable speed limits not always effective, study showsBack
Stellenbosch University|Transport|Western Cape|Christo Bester|Electronic Stability Control
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In 1999, following a fatal bus collision involving British tourists, the then Minister of Transport reduced the maximum speed limit for public transport vehicles (buses and minibus taxis) to 100 km/h. Heavy vehicles were already limited to 80 km/h.
SU civil engineering professor Christo Bester says there are a number of safety-related reasons why different speed limits could be applied to different types of vehicle.
For example, because of the differences in mass, different vehicles have different stopping distances. And, related to the inter- action between the tyres and the road surface is the ability of a vehicle to negotiate a horizontal curve. Excessive speeds can lead to a vehicle slipping, or overturning on a sharp curve.
Different vehicles also have different safety features, such as crumple zones, passenger restraints and electronic stability control.
However, despite these benefits, the application of variable speed limits is not without problems, with the most important of these being the ability to effectively enforce different limits on the same road, says Bester.
Variable speed limits are only regarded as effective when 85% of drivers keep to the designated limit. However, from studies carried out in the Eastern and Western Cape, 60% of minibus taxis did not comply with the speed limit, with the same being true for 64% of heavy goods vehicles.
Bester says automatic speed cameras cannot differentiate between vehicle types, which is why only the highest limit can be enforced.
Speed cameras that are manually operated or sophisticated apparatus with access to the South African vehicle database (such as the speed-over-distance camera) is necessary.
“It is clear, however,” emphasises Bester, “that minibus taxis and heavy vehicles do not adhere to their maximum speed limits. It is clear that these [variable speed] limits should be reconsidered or at least enforced on a larger scale than is currently the case. If the limits are not enforced, there is clearly no justification for having them.”
Bester also questions why legislation regarding the 80 km/h speed limit refers to heavy goods vehicles only.
“Why allow a higher speed limit for buses than for other heavy vehicles? Or vice versa – why force drivers of heavy goods vehicles to travel slower than heavy passenger vehicles?
“Some reason that the value of time is higher for passengers than it is for freight and, therefore, buses should be allowed higher speeds, but this is an economic decision and not one that relates to safety – the main reason given for speed limits. Economic decisions regarding vehicle travel should be taken by operators and not legislators.”
Another problem is that international research has shown that increased differ- ences in speed also result in higher crash rates, as faster and slower drivers have to be accommodated in the same road space. Particularly in two-lane, two-way roads, differentiated speed limits can result in reckless driving behaviour.
Bester says research has shown that the chance of being involved in a road acci- dent is lowest when the vehicle is travelling around the average speed of the surrounding traffic.
In the end, argues Bester, there is no reliable evidence of the safety benefits of variable speed limits. However, there is a concern that by increasing the speed variance, differential speed limits may increase overall accident rates.
“This is a solid argument against a differentiated speed limit between heavy vehicles and other road vehicles, and in favour of a standard speed limit for all vehicles.”
Bester then also suggests the introduction of a lower maximum speed limit for all vehicles, as proposed by the former Minister of Transport, as well as the further reduction of maximum speed limits on single lane roads, in line with international practice.
Edited by: Martin Zhuwakinyu© Reuse this Comment Guidelines
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