Aug 10, 2012
Variable speed limits not always effective, study showsBack
Engineering|Road|Roads|Safety|Stellenbosch University|Transport|Western Cape|Christo Bester|Electronic Stability Control
© Reuse this
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 (150 word limit)
Other Automotive News
Recent Research Reports
Road and Rail 2014: A review of South Africa's road and rail infrastructure (PDF report)
Creamer Media’s Road and Rail 2014 report examines South Africa’s road and rail transport system, with particular focus on the size and state of the country’s road and rail network, the funding and maintenance of these respective networks, and the push to move road...
Real Economy Year Book 2014 (PDF Report)
This edition drills down into the performance and outlook for a variety of sectors, including automotive, construction, electricity, transport, steel, water, coal, gold, iron-ore and platinum.
Real Economy Insight: Automotive 2014 (PDF Report)
This four-page brief covers key developments in the automotive industry over the past 12 months, including an overview of South Africa’s automotive market, trade figures, production and the policies influencing the sector.
Real Economy Insight: Construction 2014 (PDF Report)
This five-page brief covers key developments in the construction industry over the past 12 months. It provides an overview of the sector and includes details of employment in the sector, infrastructure and municipal spending, as well as insight into companies’...
Real Economy Insight: Electricity 2014 (PDF Report)
This five-page brief covers key developments in the electricity industry over the past 12 months, including details of State-owned power utility Eskom’s generation activities, funding and tariffs, independent power producers and prospects for the sector.
Real Economy Insight: Road and Rail 2014 (PDF Report)
This six-page brief covers key developments in the road and rail industries over the past 12 months, including details of South Africa’s road and rail network and prospects for both sectors.
This Week's Magazine
While Ekurhuleni-based transformer manufacturer Reliable Transformers currently designs, manufactures and tests its products according to the SANS 780 specifications for distribution transformers and other applicable transformer specifications, it is working towards...
Global endpoint security solutions company Kaspersky Lab has introduced new measures to prevent cyber criminals from accessing sensitive data, alongside its malware-signature and heuristic device analysis detection methods. Threats to mobile devices have increased...
To ensure uptake and a positive impact, Wireless Fidelity (Wi-Fi) networks in cities must be provided at schools, community centres and commercial centres to enable citizens and government to access information that will improve access to and delivery of services....
Eco-estate Monaghan Farm, located near Lanseria airport, north-west of Johannesburg, has taken a new approach to modern living and sustainability with its 517 ha development, dedicated to farm living.
Forklift and lift-truck distributor Goscor Lift Hi-Reach launched the Genie SX-180, the tallest self-propelled super boom in Africa, in Johannesburg last month. “As the official distributor of the well-known Genie range of equipment in Southern Africa, we are pleased...
Article contains comments