Thursday 21 September 2017

Stopping distances experiment #2: The real world

Last week we discussed a (flawed) stopping distance experiment, where we argued that you cannot stop from 18 mph to 0 in 6.5 metres, no matter what the police claim. That's when you are prepared to halt and planning the exact moment to pull on the levers.

What does it a real emergency halt look like? It looks like this video. Taken about half an hour before the one of a BMW driving down a pavement to get past a traffic queue, for reference.

Here is what it looks like when someone runs out in front of you while you are freewheeling down a hill (Hampton Road, BS6). Speed? Let's assume 18-20 mph. You can hear from the noise of the (hope) hubs that there's no pedalling, so this is just a gentle 5-10 mph curve round the mini-roundabout-of-death, a few spins of a drivetrain in precisely the low gear you are always in when you come up the hill from the Arches, and then coasting, relying on gravity to do the work.




  • 0:26 small kid runs out from some cars, looks like 3 parked car spaces away. Assume: 12-16 metres.
  • No previous visibility, on account of the cars being bigger than him.
  • 0:27 cyclist sees this and shouts "wooah!"
  • 0:28 bike catches up with where kid was: he's run on to be with his friends. (Assuming 16 metres, that puts velocity at 29 km/h)
  • 0:29 cyclist has now slowed down to the kids running-along-the road pace. Asks child to look. Child doesn't appear to hear them.
The entire incident is over within five seconds. There wasn't enough time to slow down before any collison would have occurred. Shouting and swerving while you slow down is all you have.

The gradient of the hill will have made stopping hard, and this wasn't the "prepared for emergency brake" setup of our previous experiment. This is real world going round the town with your hands on the tops of the levers, with gravity fighting the decelleration. The combination of the time to see and actually slow down puts the total stopping distance at something like 20 metres.

Brakingdistances.com says you for a car @ 30kmh/20mph on a -12% gradient you shoud expect 6m of thinking, 14m of stopping. Which seems consistent.

Now imagine that incident happens once a "Kim Brigg's law" is passed: a pedestrian crosses the road, cyclist > 12m away, travelling at 18-20 mph. Cyclist sees pedestrian, shouts out. Tries to veer to the side, hits the child instead. That would appear to be enough to get the mini roundabout reinstated as Bristol's Public Gallows, and your eviscerated remains left to hang for days as school parents block the roundabout in their Volvo XC90s. "Look at that cyclist, he deserved it. Now, why is this anti-car council stopping me from driving at 30, can't they see I'm late for school?"

Who is to blame here?

It's not the kid's fault he wanted to be with his friends, it's not his fault all the parked cars made him invisible until he ran out.

He didn't look. Maybe he was enthusiastic about wanting to be with his friends. Maybe he listened for a car, but didn't hear any engine, so carried on out. Children are like that: Enthusiasm is not a crime.

What did the cyclist do wrong? Well, that's a question. Is freewheeling down a hill at 18-20 mph speed limit "reckless"? "careless"? Wilful endangerment of themselves and others? The Crown Prosecution would probably argue that, while everyone from the Daily Mail to the BBC would use verbs like "plowed" and "flew" as they covered the trial. In which case: driving round the area at 20 mph, especially in a low-engine-noise vehicle (hybrid, electric) is probably even more wilful.

The one thing you can point to the cyclist and say is: you knew term time had just started, and there were other kids on the pavement. Therefore it was likely there'd be more chidren ahead. So maybe you should have braked all the way down that hill. But: no matter what speed you go down that hill on a bike. if there is a car going the same way, it'll be right behind you or trying to get past.

Which moves to a more controversial question: is 20 mph too high a speed during school start/finish times? Should we drop from 20 mph to 15 in areas near schools? For everyone, drivers and cyclists alike?

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