Recent research by the Transport Research Laboratory (TRL) in the UK, has shown that all may not be well with some speed hump designs.

After complaints of excessive noise, vibration, increased vehicle emissions, and significant increases in accidents involving cyclists and motorcyclists, the TRL have confirmed that where truck traffic exceeds 10% of the traffic stream, these problems are likely to become significant. (Ed. note: there may be some difference in these figures for North America due to different vehicle parameters.)

In one survey of residents in a town where there were significant complaints, the TRL research showed that 75% of residents thought that noise levels had increased even though traffic was slowed down by the humps.

Apparently original estimates of noise reduction had ignored the effect of trucks and buses on noise levels. Revised analysis of emission levels also indicates increased levels of emissions as vehicles brake and accelerate.

Review of geotechnical issues indicates that soil conditions also play a significant part in the transmission of vibration into building foundations and superstructures, the worst effects being on peat soils.

The TRL has reduced their recommended speed hump design from a 100mm rise to 75mm, and reduced the maximum ramp slope.

TRL is now recommending that Road Authorities consider other devices to slow traffic that do not result in these vibration impacts, engine noise and emissions problems. These include chicanes, and textured surfaces that produce high tire noise at high speeds.

Researchers are also working on a "high tech" speed bump composed of a "liquid" filled bag covered with a protective matting that can be bolted in place on the road surface. The "liquid" (similar to that used for tissue implants) is pliant and takes a low profile under slow speed wheel impacts but becomes significantly firmer with higher speed impacts, above 40kph, and rigid at 50kph. The effect is to minimise speed related vibration to those vehicles that are travelling above those speeds. This could give a "whole new feel to driving" said a TRL spokesman.

(This item is based on an article by Jonathan Leake in The Times, London on December 28 1997.)