Anatomy of a Speed Hump

This article was published in 1998, in Newsletter 18.

Sleeping policemen, rumble strips, speed bumps, call them what you will, they can at the same time make life both easier and harder for cyclists. Easier because they slow down motor vehicles (in theory), and harder because they inconvenience cyclists. And that’s because they’re largely the wrong shape for a cyclist’s thin, large-diameter wheels. Occasionally one might want to slow cyclists down too, but usually this is a side-effect, and cars are slowed only to speeds significantly higher than cyclists would be travelling anyway.

But there’s speed humps and speed humps – and there’s some that aren’t humps at all. And of course humps aren’t the only way of slowing down traffic.

Cyclists’ safety depends on slower traffic. Sadly, the majority of motorists can’t be trusted to limit their speed – they even admit to it. In a survey last year, 70% of motorists admitted breaking speed limits, and government research bears this out with a very similar urban figure. So in residential streets, they need to be forced to slow down. At 20 mph, reaction time gives more chance to avoid a collision and, if a collision does happen, 5% of pedestrians and cyclists die. At 40 mph, 85% die in a collision. Slower speeds are crucial.

Where are the best speed humps in Cambridge? In my opinion they are in Ridgeon’s builders yard, off Cromwell Road. These are high enough to be effective in keeping speeds down, they are perfectly constructed, unlike many on public roads, but most importantly they are ‘sinusoidal’. This means that in profile you meet them on a rising curve, not at an angle. As well as being much more comfortable for cyclists, this also makes them quieter when cars meet them – an important consideration of acceptability to residents. So why aren’t they used elsewhere? Obvious really – the Department of the Environment, Transport and the Regions construction standards only recently permitted this design, and don’t encourage its use.

The more familiar ‘slice off a circle’ profile is exacerbated in Romsey by the uncomfortable (imitation) setts. The smooth tarmac versions in Stretten Avenue, Ditton Fields and many other places are slightly better.

Then there are so-called ‘speed tables’, where an extended hump has a flat top. These are often used across junctions. They’ve been placed at minor cross-roads, for example at Rock Road and Stretten Avenue; and on the cycle track at Milton Road, and are shortly to be put in on Barton Road to carry the track across side roads. They are also used in traffic calming, for example in Cherry Hinton High Street and outside Morley School in Blinco Grove.

On the whole I quite like these, perhaps because they are sited typically where you need to slow down on a bike too. But they typically have a straight slope up, and if this is badly constructed, as those at Blinco Grove were originally (they have recently been repaired), can be extremely uncomfortable.

A relatively rare feature in Cambridge is the ‘speed cushion’. This is like a small speed table, but it does not extend right across the road. They’re wide enough to catch cars, but leave a big enough channel so that cyclists can avoid the hump completely. Only Arbury Road in Cambridge has these to my knowledge.

Image as described adjacent
in Ridgeon’s off Cromwell Road
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a Romsey Town hump
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speed table in Stretten Avenue
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speed table in Blinco Grove
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a rare speed cushion in Arbury Road
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a speed hump in Ditton Walk

So the message to the engineers is simple – use speed humps that reach their target, without the cycle-unfriendly joints between road and hump. Unfortunately, this discomfort can sometimes mean cyclists use busier roads in preference, possibly putting themselves at greater risk.

David Earl