And of course death. Those in favour of increasing assist speed to 20 mph are perhaps not considering the deaths caused by cyclists at that speed. A few months ago a woman was killed by a 20 mph cyclist, the case gaining notoriety because the bike had only one brake. Not long before that a man crossing the road from behind a parked car was hit and killed by a 20 mph cyclist, who a court ruled had no chance to avoid him. And also very recently a cyclist lost stability on some loose gravel in a 20 mph limited Royal Park and was killed when he fell off, despite wearing a helmet.
When do we ever hear of pedestrians killed by 15 mph cyclists, or cyclists killed by falling off at 15 mph or less? The fact is that stopping, slowing down sufficiently or saving oneself in a fall is so much easier at 15 mph than 20 mph, and the weight of bike and rider is far less damaging at the lower speed.
If there was a change to 20 mph, the performance would be used, inevitably the number of deaths would rise, and the equally inevitable outcome would be more restrictions like registration and insurance, with use of shared pavements and cyclepaths etc. banned.
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