This blog has moved

This blog has re-located to Chester

Thursday, 4 August 2011

Pedestrian danger from cars versus cycles

Whenever cycling is discussed in the mainstream press, a common theme which often emerges in the comment thread following the story is the perceived danger cyclists pose to pedestrians. A good example of this is the comment thread under any of the stories published in The Grauniad Bike Blog. It is a truly impressive feat that the motoring lobby has managed to pull off; persuading pedestrians that it is cyclists, not motorists who pose the biggest threat to their safety. 

These comments are usually countered by comments from cyclists themselves, who are quick to point out the much, much greater risk posed to pedestrians by motorists. Usually, these comments consist of a series of suppositions which, whilst often perfectly plausible, are difficult to back up with actual documented evidence. These suppositions often include the following: Cyclists are less likely to be distracted on the road compared to motorists due to being aware of their increased vulnerability in the event of a collision, cyclists are more likely to notice pedestrians in their vicinity because their visibility is less impaired than that of someone sat in a car, cyclists are exposed to fewer distractions on a bike than motorists are in a car (generally bikes don't have radios/heaters/fans or passengers), the nature of cycling makes it less feasible to try to text/shave/apply make-up whilst cycling when compared to a motorist sat in a car. Please feel free to leave any other reasonable-sounding suppositions in the comments below.

Another argument given is that cyclists generally travel at lower speeds than motorists and that a bike & cyclist together are smaller and lighter than a car. Unlike the suppositions above, these things are either demonstrably true, or obviously self-evident. The figure at the top of the post shows a diagram of 70 metres of a 6 metre wide road with 1 metre pavements either side. The left lane shows a blue bar, graded in increasingly dark shades. The lightest section of this bar represents the space a 2 metre wide motor vehicle (such as those models favoured by pimps) travelling at 30 mph would travel through during a 3 second "Distraction period," during which its operator is not devoting their full attention to the road. The vehicle travels approximately 40 metres during the 3 second window where the driver is not paying attention, meaning the vehicle will cover an area of approximately 80 square metres. Any pedestrian (or other road user) present in this area at the time the driver takes their eyes off the road is at risk from the motor vehicle. At 40 mph (darker blue), the car will travel approximately 53 metres during this distraction period, covering an area of approximately 106 square metres. At 50 mph (darkest blue), the car will travel approximately 66 metres during the distraction period, covering an area of approximately 132 square metres. Speeds of up to 50 mph are still relatively common on urban roads, these speeds even being officially sanctioned on some roads.

The right lane shows a pink bar, graded in two shades of pink, representing the area which would be covered by a typical 0.6 metre wide utility bicycle travelling at 12 mph and 20 mph (light pink and darker pink respectively), during the same 3 second distraction window. At 12 mph (a fairly typical cycling speed for a utility cyclist) the cycle will travel approximately 15.8 metres during the same 3-second distraction period, covering an area of approximately 9.5 square metres. At 20 mph (the sprint speed recommended in Cyclecraft to enable cyclists to reasonably deal with our fundamentally cycling-hostile road network), the cycle will travel approximately 26.4 metres during the distraction period, covering an area of approximately 15.8 square metres. For a sense of scale, the black rectangle in the lower left corner is 1.7 metres by 0.6, representing the height and width of a relatively large human.

So there we have it, at a fairly common urban speed of 30 mph, a car will travel over an area more than five times greater than the area a cycle would cover at the relatively high speed of 20 mph during a three second period of operator distraction. At a typical cycling speed of 12 mph, a cycle will travel over an area which is over 8 times smaller than the car would at 30 mph. This representation is a bit of blunt instrument, It'd be great to see some research which takes into account the effects of vehicle mass, vehicle profile and some of the suppositions listed above too. As it is, I think it helps to hammer home the fact that it is of course the private motor vehicle which represents the greatest risk to the safety of pedestrians (and cyclists, and even other motorists). The reason people focus on the (much, much lower) risks posed to pedestrians by cyclists, is that the private motor vehicle is the sacred bull in society's china shop.


  1. It also always amazed me that while people feel there's a need for segregating cycles from pedestrians for safety of the latter an idea of separating cars from cycles isn't as popular.

  2. The other piece of evidence is that in an average year two or less pedestrians are killed by motorists, whereas 80 pedestrians are killed by motorists driving on the pavements, and there are the pedestrians are killed by motorists as they try to cross the roads...

    Maybe this is why 20% of the population don't say they take walks of 20 minutes, less than once a year or never. Also, that in 2010 20% all trips less than one mile (1.6 KM) in length were made by car.

    So maybe this is why Government minsters keep telling us that they can not introduce any policy that which would restrict car use as "many people rely on their cars". All of which is a problem when we desperately need to "say no to ridiculous car trips, and use active travel instead...

  3. Errmm, you seem to be saying that a car travelling at 30 mph will travel 40 meters in 3 seconds, but a bike (and presumably a car, a helicoptor, or a hovercraft for that matter) travelling at 20 mph will travel 8.8 meters in the same three second period? Or have I misread?

    I'm interested in how you've calculated this as it sounds like it's pretty obviously wrong - something going approximately a third faster than something else is going to travel about a third further than the other thing for any given period. I make the 20 mph object will travel means a mile every three minutes, so 1600m/180 seconds x3 = 26.6 meters travelled in a three second distraction period.

    Sorry, don't mean to nitpick, as the basic point (things going slower are less dangerous than things going faster) is entirely sound - it's just your figures don't make any sense to me.

  4. @Tim,

    The diagram was fine, just the numbers were off (cycle numbers were speed in m/s, rather than distance covered in three seconds). Fixed it now, thanks for pointing it out.

  5. Mr C, you neglect to incorporate braking and more importantly, reaction distances.
    Bringing one's attention back to the road and spotting a hazard ahead, first one has to react, then brake. Therefore, the increase in total distance is a squared multiple of the increased speed.
    An example, a car at 30mph suddenly faces a hazard - the driver reacts, brakes and stops just short of the hazard. The same car starting at 37 mph would still be travelling at 22 mph when they struck the hazard! The forces on a pedestrian being struck at 22 mph are the same as falling onto concrete from the roof of a 2 storey house.
    If you incorporated braking and reaction into your diagram, I fear you wouldn't have room on the page for it.

  6. Great, makes much more sense now! Thanks for clarifying.

  7. @crapbournemouthcyclist,

    I did consider adding reaction distances based on an average person's reaction time. I also considered the issue of stopping time, but things quickly become very complicated. Whilst there are certain standards in place for vehicle braking, there is considerable variation. This holds particularly true with bikes; many bikes have excellent braking (particularly bikes owned by enthusiasts), but there is a huge amount of variation between different brake types and configurations. Add into the mix the current trend for brakeless fixies and it starts to get very difficult to add onto a diagram such as this. One thing I will say on the matter is that the ability of bikes to swerve to avoid danger (both due to their small width and hugely increased manoeuvrability compared to a car) would make people on bikes less of a threat to pedestrians.

  8. Re:'cyclists are exposed to fewer distractions on a bike than motorists are in a car'

    I have seen a woman on the motorway brushing her hair with both hands, while driving on the motorway. A man engrossed in texting, while driving on the motorway.


    As far as I can tell, no cyclist has ever been seen using two mobile phones while travelling at 70 mph.

  9. The other thing to mention is that I think a bicycle can generally turn and dodge to avoid a collision much quicker than a car. Also the fact that the cyclist will be hurt as well in a collision with a pedestrian also creates a built-in deterrent to avoid collisions in the first place.

    Great blog by the way.


This blog has moved to Chester. All the old posts can be found at Chester Cycling where I invite you to continue the discussion instead

Note: only a member of this blog may post a comment.