My reference shows 1% at 30km/h. But even 1% is too high. But luckily you also usually get a chance to scrub off some speed through braking. Braking follows a square law, so driving a little bit slower gains a massive difference in stopping distances.
If you're ever driving in a situation where it is possible for a nun or baby carriage to jump in front of your car without any warning, you should be driving at 30km/h (20mph) or less. This gives you a stopping distance of approximately 1 car length, so stopping is always a better option than trying to steer around an obstacle. It also means that any collision with a pedestrian is highly unlikely to be fatal.
Of course, having automated cars driving at 30km/h through the suburbs is going to piss everybody off, but that's better than dead babies.
Breaking distance and collision energy scale with square of the speed. So reduction from 70 to 60 is 27% reduction.
Also minimal safe distance is 3 seconds at your current speed. If you can't keep that distance because of traffic - you should slow way down, not to 60 km/h but much lower. So either your comparison isn't fair or the person doing 60 km/h with 2 seconds of gap is the reason the road is unsafe, not the traffic.
The momentum point is very valid. But the slower speeds provide more time for drivers and others to react. That in turn gives you much more time to break and scrub off that extra momentum.
At 50kph your reaction distance is about 15 meters, at 25kph it’s half that at 7.5meters. (Assuming reaction time of 1 second, which seems to be a realist value).
7.5 meters is quite a bit of distance, and in a busy city will make a lot of difference.
Yeah, you should wait. The streets are for pedestrians and cyclists, car drivers can do whatever they need to do later. You can accelerate from 0 to 60 miles per hour in 5 seconds. We're walking around at 3mph if we're in a hurry.
Not bad, but you need an element from the square of velocity (for kinetic energy/braking distance) there to be safe. 1 second is fairly okay at low speeds, but at 60 mph (~100 km/h, 27 m/s) two seconds is shortish and if you are doing 200 km/h (as on a German Autobahn) then 2 seconds (100 m) is not safe in my opinion.
(Although the penalty line in Germany is just this, half the speed in km/h to get meters, i.e. driving closer than 100 meters if going 200 km/h gives you a ticket for tailgating)
>Recommended following distances are nowhere near "instant stop of vehicle in front of you plus reaction time".
But the nothing can instantly stop, it's physics. The recommendations, at least in Germany are so that you have enough time to react + brake if the car in front of you does an emergency braking manœuvre.
If the car merges into your lane at too little distance, like here, you have to slow down. A lot of people don't.
You shouldn’t need data, it’s common sense. How long does it take a car to go from stopped to 30mph? Five seconds? How long does it take a cyclist to go from stopped to 10mph? Certainly not less five seconds. In those scenarios, average acceleration for the car is 6mph/s, for the cyclist it’s 2mph/s.
It's not just about reaction time; 30 km/h is the limit where you can collide with a pedestrian and be fairly assured that the pedestrian will live. Above that speed survival rates drop dramatically.
I leave quite a reasonable distance. The issue to me is it slows down before that with no cue, so I occasionally look down to find i'm somehow going 20km under the speed limit, a hundred meters behind whatever is in front of me.
True of course. At low speeds, the braking distance is negligible, and the margin is needed for driver reaction. When exceeding 60-80 km/h, the amount of kinetic energy and correspondingly the braking distance will start to dominate what is necessary for safety margin.
> Allow at least two seconds' following distance behind other vehicles in good weather and road conditions (three seconds on a highway).
> Slow down for poor weather conditions or uneven roads and increase your following distance to at least four seconds. Remember that the distance required to stop increases in wet or slippery conditions.
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