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Every now and then when I ride over a peanut sized stone the tires squeeze and accelerate it to the side at up to 30m/s. So far I have only hit a building and a car which ended up with a scratch. Am I responsible for the scratch? What if someone got injured?

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    What are the rules where you live around stones flicking off truck or car tires. Where I live the driver of the car that flicked up the stone is not normally considered at fault, and the driver of the damaged car may have recourse to the roading authority (especially if there is recent road works). However, requirements for safe driving mean police may take action if it was obviously going to happen. Normally however its considered 'just one of those things' – mattnz May 31 '17 at 19:34
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    How did you establish the speed of the stone? – gschenk May 31 '17 at 21:58
  • I tend to avoid riding through patches of stones - they can unstick a road tyre which is disconcerting. Single stones on their own tend to stand out on smoother road tamac, but are invisible on chipseal. I'd call it a danger of being on the road, and the cyclist is only liable if they're doing it on purpose - which would be very hard. – Criggie Jun 1 '17 at 1:09
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    +gschenk I guessed the time difference t between your tire hitting the rock and the sound of the rock hitting an object. Then I measured the distance between tire and struck object d with tire revolutions (Circumference of circle = 2πr). Now that I have d and since the speed of sound is not infinite I can correct t. I calculated the speed of sound v at my current temperature and altitude using this calculator: sengpielaudio.com/calculator-airpressure.htm. The correct time difference now is T = t - v * d. To get the speed of the stone s you do: s = T / d – AzulShiva Jun 2 '17 at 16:49
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There are three parties involved:

A. You (the bicyclist)

B. The driver

C. The local/city/state/federal agency responsible for road maintenance

If the road was in good condition and the car was following too closely to you, then the car might be at fault. That being said, in the USA, many states require that windshields be covered under the 'no-fault' portion of the insurance, so the driver will have it replaced by their own insurance company.

If you were riding in a manner either negligently or deliberately that might kick up rocks, then you might be responsible.

If the road was poorly maintained and you and the driver were both acting responsibly, then the city/state/local agency may be responsible - although many times they have immunity.

Which party is responsible is determined by the situation as well as the traffic laws of your country. Ultimately, a jury or administrative judge would figure out where to find fault if it came to that.

In the case of a building, since it obviously can't move, you might be the responsible party unless you can blame the city/state for the poor conditions of the road (an argument which would be weakened if you commuted regularly on that section and thus knew that the conditions there were poor). Whether you are liable and for how much again depends on the laws of your country.


Note that you don't ask how to avoid this, but here are some thoughts:

  • Increase or decreases tire pressure. There's a sweet spot in which tires pling rocks. Either increasing or decreasing your tire pressure may put you out of the plinging zone.

  • Get wider or thinner tires. Same as above.

  • Slow down or speed up.

  • Avoid that section.

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    Please note: The stone is travelling at 30m/s not 30cm/s. I've actually had them hit a building on the right, ricochet, narrowly avoiding me while whizzing across the street and hitting the building on the left... with a lot of force left!! I'm talking almond sized ugly little stones with sharp edges that could travel well over a hundred metres while spinning like a neutron star. It's like a BB gun bullet only much heavier and out of control. I'm surprised this has never happened to you cos it happens to me all the time. – AzulShiva May 31 '17 at 18:28
  • If there is a portion of the trail where this happens regularly, prudence would dictate that you would slow down for those portions lest you hit a bystander (or car). If you knowingly enter a portion of trail where this happens and it happens, then you would likely be found to be at least partially at fault. – RoboKaren May 31 '17 at 20:40
  • It happens pretty randomly and only on asphalt. With "all the time" I mean maybe once every 1000km. If I ride on gravel they do jump but not nearly as far. – AzulShiva Jun 2 '17 at 16:28
  • Then you really can't predict it, so it's not your fault. – RoboKaren Jun 2 '17 at 17:16

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