1989-11-01

The In-Service Braking Levels of Australian Heavy Vehicles 892505

A lack of knowledge of driver behaviour and vehicle performance at everyday braking levels has made it difficult to develop Australian Design Rules which achieve a balance of braking between heavy vehicle tractors and trailers under all operating conditions. This paper examines the in-service braking behaviour of heavy vehicle drivers to provide information for use in the development of design rules, which would improve tractor/trailer braking compatibility under routine braking regimes. Results show that while braking is required every 500 m under heavily trafficked urban driving conditions, on roads with high levels of service this can reduce to once every 11 km on average. Under the right road and traffic conditions, heavy vehicles were shown to travel up to 90 kms between brakings. Average braking duration was between 6 and 9 seconds with average deceleration rates of between 0.4 and 0.7 m/s2. These decelerations are considerably less than the 0.25 g accepted as the limit of comfortable passenger car braking, as well as those within which the current Australian Design Rules for truck braking are framed. The average speed change during braking was found to be small for all road types considered, with very few brakings made from very high to low speeds. The constant adjustment of speed to the conditions by drivers appears to pre-empt the need for severe braking. The majority of pedal pressures were found to be below 100 kPa and the rate of pressure rise was modest compared with expectations. Less than 1% of brakings showed the driver to be modulating his braking during the deceleration. The profile of normal braking behaviour developed by the research has allowed recomendations to be made for changes to the Australian Design Rules for heavy vehicle braking and these are currently under industry review.

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