How are speed limits of roads determined?
In: Engineering
12 mph- pretty much anyone survives anything at this speed- so high interactions and manoeuvring like car parks – 10mph is the round number limit.
30mph – this is the speed at which the human body spins when a car hits it. Below 30 the car pushes the pedestrian away. Above 30 the human body spins on its centre of gravity and the head hits the engine block – massive damage – high risk of death or serious injury. So where high risk of car vs person then 30 should be max.
About 90 mph is the survivable head on collision between two vehicles- so if no pedestrians and single carriageway – it should be lower than 50- to give the closing / head on max impact speed of about 90mph.
Similarly on highways / motorways – but as all cars going in same direction with little head on risk – speed limit should be about 90.
There’s a quirk at 55mph on highways as that’s both the speed at which fuel economy is best and all vehicles – trucks. Buses and cars etc can all travel. Accidents on highways/ dual carriageways happen when people pull into a faster lane to overtake and a car coming up. Behind is travelling much faster. This causes heavy breaking – bunching and then accidents. So if all vehicles are travelling the same speed even when some jerk pulls out randomly – this bunching is less prevalent – so there’s a good argument for 55 mph on highways.
In the uk if the speed limit is anything other than 30mph where there are street lights or 60 mph where there are not street lights then the local authorities have to pay for signage indicating speed limit every 200 yards. So often the speed limit is set to these limits regardless of safety just to save money on extra signs
Any other speed limit is just made up as far as I can tell. My source is Thatcham the uk accident investigation centre – but I don’t have a link
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