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A vehicle driver must keep enough braking distance (aka stopping distance) between him/her and a vehicle in front in order to avoid collision. It depends on the driving speed.

What is the braking distance for bicycles:

  • between two bicycles?
  • and between a bicycle and a vehicle in front?

Is it calculated with the same formula or with a special one?

And which countries enforce this rule on cyclists?

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For a bicycle it depends a lot on how aware the two riders are of each other. The rider behind, eg, can watch the feet and hands of the rider in front, and even watch the brake calipers, to see when the rider in front may be slowing. The rider in front, of course, needs to not make any terribly sudden moves. –  Daniel R Hicks May 1 '13 at 10:59
    
From a strictly legal and safety point of view it must be the rider behind's responsibility to keep a safe distance. You can always go slower, you can't always go faster. Of course there's also some common courtesy and an acceptance amongst most of us of the benefits of drafting. –  James Bradbury May 1 '13 at 11:23

2 Answers 2

up vote 4 down vote accepted

The stopping distance is a factor of:

  • Reaction Time
  • Speed and Mass (bike, rider, and load)
  • Efficiency of Brakes
  • Braking force applied (on which brakes and how applied)
  • Road/track surface (including water, ice, gravel, manhole covers etc aspects)
  • Tyre width, grip, tread etc

You have to notice a need to brake, move your hands to apply braking force. Then physics comes in, a bike of a given mass moving at a given speed has a certain amount of inertia that needs to be overcome and how much you squeeze the brakes.

So it's going to vary massively by bike/rider/situation.

The same is true of cars. Top Gear (UK) demonstrated a few seasons back the UK Highway Code approved braking distance at 60mph for a car, then showed how a reasonable car could go from 100mph to a full stop far, far, far short of that distance.

It's trained as a rule to car drivers because of the significant damage that can be caused with a big heavy lump of metal moving fast, based on very old figures.

I doubt any country enforces it on cyclists. And road cyclists in cycling groups routinely ride on each other's wheels to get the benefit of drag etc. There training and skill will come in.

Generally, the practical rule trained out in the UK for motorists (and I've been (un)fortunate enough to be required to attend a driver skills course following an incident plus opted to attend some advanced motorcycle training) is the 2 second rule.

Keep a gap of 2 seconds between you and the vehicle in front. That allows you to stop in a controlled fashion. if someone is sat close behind you, expand your stopping zone to buffer their lack of stopping zone. Increase to 4s in the wet and 20s in icy conditions.

On top of that, when it comes to bends, always make sure that you can stop in the space that is visible, so you naturally have to slow down as the road bends.

That said, keeping a 2s gap in a cycle commute is going to have the car behind you putting you at risk!

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Your 4 bullet points are good but you need to add two more: 5) terrain 6) rider skills –  cherouvim May 1 '13 at 11:07
    
Yeah just about to answer the same. I would also say the road surface and tyre width play a big part too. And of course the classic are you using front or back brake. –  will May 1 '13 at 11:09
    
There are many variables, that's why it's an interesting question, IMO. Also consider the length of the bike and position of the weight. Even without insulting my wife, there's no way the rear wheel of our tandem is going to lift! –  James Bradbury May 1 '13 at 11:21
    
@JamesBradbury Yeah, Remember to be careful depending on which bike you're riding. On my commute I usually have a rear pannier with 10-20 lbs of stuff in it. When it's not there I definitely feel the difference when breaking, and notice that the back wheel lifts very easily. –  Kibbee May 1 '13 at 12:51
    
Some good points, made some updates. Skill of the rider is going to be in how they apply force to which brakes. Have to be progressive, start slow and build up. Don't want to lock the wheels. particularly not the front :) –  THEMike May 1 '13 at 13:46

I'm not sure if you're referring to specifically road riding, but most of the comments have been mostly related to street situations. If you are in a mountain biking / trail riding situation, it's also important to factor possibility of accident into your stopping distance--which is always important, but can be more important when mountain biking. You might be at a distance that it is easy for you to stop when on a street and be way too close if your buddy crashes in front of you, both because of the inherent difficulties of the terrain and the additional obstacle that your buddy now provides.

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