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.
Speed determines time to stop including stopping distance and reaction time. For every second of reaction time per 10kmph you add about 4 meters of distance travelled before you even hit the brakes. So best case if you aren't looking at your HVAC, at 50kmph you will start the stopping process in about 28 meters, and at 60kmph it will be 33.3m and take longer to stop once you hit the pegs.
Doesn't sound like heaps but that is actually your margin for error, just 4m. If something jumps out in front of you at 28 meters away, at 50 you will have had 4 meters to stop, which modern cars can do, and at 60 you will hit them at the full 60kmph. The Australian government has some great road safety clips on the subject, I might try and find them.
Even if there is no reaction time, 5' of space at 50mph is silly. You still need room to maneuver if whatever caused the vehicle ahead to throw on its breaks comes back or halts the vehicle ahead faster than your breaks can stop you.
Probably in the sense that at a slower speed, op would have been able to avoid the bad driving from the other driver. Assuming that OP was driving at 60kph originally, a 15kph slower speed equates to a roughly 0.75 extra seconds of reaction time (assuming braking acceleration is roughly 7.5m/s2). Given that an average humans reaction time is somewhere between 0.25-0.5 seconds, that means 15kph speed difference would have allowed OP to react and affect the situation. Even at higher initial speeds, 15kph gives your more than half a second extra time.
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.
As humans we know our reaction times - you naturally slow down as you get to a junction or navigate crowded or tricky traffic conditions to give yourself as much of a chance as possible to react.
This is the thinking behind many types of speed control street layouts. You should also know where to look to anticipate where danger is most likely to come from, and be ready with some kind of action. This is why we do
hazard identification tests as part of the driving test - looking in the right directions at the right times is crucial for operating a vehicle safely.
250ms is a fairly average reaction time for something visual that you are ready for - but you should really be giving yourself as much time as possible - if somebody bombs past a traffic light at 70mph, even if it is green, most people would agree that it was an unsafe move. This goes doubly for an autonomous car, that is unable to play the positioning negotiation game that humans are masters of as a result of being social creatures.
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.
50 ft is 15 meters and change, at regular car speeds that is much shorter than your reaction speed. It's essentially an accident waiting to happen. At highway speeds you should be looking about 200 ft ahead of you for safe driving. See the 'two second rule', and that's for clear skies, daylight:
Good driving doesn't have that much to do with your reaction time. 40% slower to respond, with a well planned drive, is a lot safer than 40% faster driving too close.
It's not half a second in the context of minute-long trip, but rather in the context of driver reaction time which is (IIRC) around a couple of seconds. So, it's substantial.
It is not just a matter of reaction times. It is mostly of stopping without harming you, which at 70mph is rather difficult if the vehicle in front brakes too hard.
!!!! 2 meters separation at 70 km/h gives you a tenth of a second to react to anything the car in front of you does, that's flatly insane. Where in the world do people drive like that?
Seriously, that's objectively insane. Try the ruler drop test for reaction times if you don't believe me, a 10th of a second to even initiate your response isn't realistic and obviously gives no time for the response itself to have effect. What I'm saying is that at a tenth of a second, you can't even start to press the brake pedal in time, let alone have enough time to actually slow down.
In America, with only 2 meters between vehicles the traffic would be inching forward at a snails pace, under 20 km/h at least. "Stop and go", as in people would stop their car and then drive forward slowly when a larger gap ahead of them appears.
The effect is huge. Energy goes up quadratically, so here (4->9) you have more than doubling. Stopping distance goes up quadratically (well, due to reaction time there is a linear component in there as well, but still).
And if there is a sudden obstacle that you can just avoid hitting going 20mph initially, you'd hit it with 22 mph if you're going 30 mph initially (assuming no reaction time and constant deceleration).
I submit that that's a massive difference: hitting a child not at all or with 22 mph.
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.
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