Devices, commonly called 'spacers' (attached to the bicycle to discourage drivers from passing too close to the cyclist when overtaking) have been evaluated. Commercially available spacers usually consist of a plastic rod with either a flag of disc attached at the end. The overall length is about 0.4m. Spacers are usually mounted horizontally on the rear offside of the bicycle so that they project toward overtaking traffic. They are widely used in Scandinavia but have only recently gained acceptance in the UK. By employing a specially developed ultrasonic ranging technique that allowed the easy measurement and recording of large numbers of gaps left by drivers overtaking a test cyclist it was possible to quantify precisely the effects of spacers of various types of drivers' overtaking behaviour. Measurements were taken on a variety of roads during the day and night. Results show that the percentage of overtaking vehicles passing less than 0.8m from a test cyclist was approximately halved when using a spacer 0.5m. in length. A spacer 0.35m long was only about half as effective. The effects of the 0.5m long spacer were shown to be very similar to that of a fluorescent yellow jacket for discouraging close overtaking.

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