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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. 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. May 1 '13 at 11:23
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    That's not the braking distance, it's the reaction distance: Two cars driving at the same speed have roughly the same braking distance, so if both drivers slam the brakes at the same instant, even a minimal distance would be sufficient to avoid collision, no matter the speed. However, the driver of the following car cannot react instantaneously to the braking of the leading car. As such, they must ensure that they can react before they pass the point at which the leading car slammed the brakes. That's the reaction distance. Jul 25 at 10:15
  • To give a few numbers by applying the thumb rules: A car driving at 180km/h (= 50m/s) has a braking distance of about 18*18 = 324m, but a reaction distance of only 60m. If you do any driving on a German Autobahn, you'll know that those 60m are much, much closer to the actual distances between cars (the side posts are 50m apart, and that's on the same scale as the actual safety distances). Jul 25 at 10:23
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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! 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
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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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between two bicycles?

Assuming the two bicycles have good brakes, this is solely caused by reaction time. The bicycle in the rear must be able to quickly determine that the bicycle in front has started emergency braking. I'd say 2 seconds of distance is a good idea. At 35 km/h, this is bit less than 20 meters, or bit more than the length of ten bicycles in line.

We can already from this see that racing cyclists do not observe this distance, mainly due to getting aerodynamic benefit from riding close to the cyclist in front. This gives an explanation that when one racing cyclist crashes, several dozen in the rear crash too.

and between a bicycle and a vehicle in front?

Assuming the vehicle is an automobile, they have 1.0 g braking power when braking hard. Bicycles, on the other hand, are limited to only 0.6 g braking because with harder braking the rear wheel rises into the air.

You must also include the 2 second reaction time.

The car at 35 km/h (assuming a low speed for the car because otherwise the bicycle wouldn't stay behind the car) brakes in 4.8 meters, practically instantly.

The bicycle on the other hand brakes in 8 meters.

So to the 20 meter distance you anyway need for reaction time, you need only add 3.2 meters of distance due to unequal braking power. So it's 23.2 meters. Not much different from 20 meters.


Also do note that rim brakes when wet have practically no braking for two wheel revolutions until full braking power kicks in. So, with rim brakes in the wet, add 4.3 meters to the reaction time. Disc brakes don't need this. Rim brakes also usually don't need this because cyclists using rim brakes in the wet usually anticipate braking and begin applying the brakes lightly to displace water from the rims immediately before braking might be needed. However, for sudden unanticipated stops the 4.3 meter penalty applies.

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  • 1
    Nitpick: The deceleration of a bike depends on the frame's geometry. Most bicycles have a rather short distance between the wheels, putting the center of gravity at a steep angle above the front wheel; this geometry invites the danger of going over the bars. However, it is possible to build bikes with a longer wheel distances and a much shallower angle between front wheel and CoG; I happen to own one that actually makes it impossible to go over the bars, the front wheel will slide over the road surface when it blocks up. Thus, max braking deceleration of this bike is on par with cars. Jul 25 at 10:34
  • I agree, I assumed a traditional bike. Some recumbents may have far better braking.
    – juhist
    Jul 25 at 10:49
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    @cmaster-reinstatemonica Thus, max braking deceleration of this bike is on par with cars. Only if you can modulate the brakes on that bike so you get maximal braking without locking up the front wheel. Almost all automobiles now have antilock brakes so can easily push into the 0.9 to 1 g range of braking deceleration. A locked wheel with a rubber tire sliding across the road surface is probably going to be in the 0.7 g range. Jul 25 at 11:24
  • @AndrewHenle Yeah, initially it may be less. But it becomes more when the rider hits the road... Seriously, of course I must keep the wheel turning, and yes, I don't have an ABS built into my bike ( :-( ), so cars can still outbrake me. Nevertheless, I can get much closer to car braking performance than if I were riding a modern racing bike. Jul 25 at 12:50
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About 3 meters on a reasonably careful ride (below 25 km/h), but after the brakes have been applied (response distance). More like 25 meters if we add the 2.5 s time required to spot the danger already in sight and press the handles, but 4 meters of we assume a very vigilant driver with 250 ms reaction time. See the charts below for details.

The formula (found here) states the reaction distance (brakes applied) it is more or less equal to V^2/254. I have calculated the chart:

Brake distance, reaction time

However this is only about your bicycle, not yet about you. The source states it may take about 2.5 seconds for a cyclist to spot the danger in sight and apply the brakes. This second distance is calculated as V/1.4 in the source and makes the total braking distance much longer:

Brake distance reaction time 2.5 s

But as personally to me, 2.5 seconds looks like a very long time to react, unless you are listening over radio braking news that change your life. The reaction time of the highly concentrated human is about 250 ms (another source here), so ten times less than this formula assumes. As the "reaction" member of the equation likely assumes rolling with the current speed for duration (brakes not applied yet), it is possible to recalculate the chart for the vigilant, alert driver:

Brake distance, reaction time 250 ms

This last chart looks for me more realistic estimation comparable also to what I would expect from my experience. However it is computed combining the two sources and extrapolating the existing equation so please interpret with care.

Of course, all this depends also on your mass, brakes, road, slope and the like. The formula probably provides some "generally average" values. Main conclusion of the work is, likely, do not sleep while driving a bicycle because your reaction time is the major component. If the two bicycles travel behind each other, it is the reaction time that causes the collision as both would otherwise just take the same distance to stop. 250 ms is likely only possible if the front driver gives some very clear sign about braking.

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  • 2.5 seconds sounds unreasonably large. If you pay such little attention to the road in front of you, you shouldn't be on a bike at all. Car drivers generally use only about 1 second of safety distance (which is indeed too little, imho), and mostly manage to brake in time. If you give yourself 2 seconds, you are already very relaxed about braking, anything more seems unnecessary. Jul 25 at 10:41
  • I have recomputed the version assuming 250 ms reaction time found in another source. This one looks to me more realistic.
    – h22
    Jul 25 at 12:04
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    Well, 250ms is the time you need when you are fully committed to slamming a button (= brake) when a signal lights up (= brake lights). It does not leave any time for modulating your response. That's why typical car safety distance rules use either a 1s rule, divide the speed in km/h by three (= 1.2s rule), or multiply it by 0.3 (= 1.08s rule). This allows a driver to first gauge the deceleration of the leading car before slamming the brakes. Bike riders should err on the side of caution, so 1.5s to 2s rule should be appropriate. I would definitely never suggest that 0.25s can ever be enough. Jul 25 at 12:45

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