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 stopping distance at 60mph/96kmh (~28m/s) for a typical car seems to be 133ft/40m. So you are right, it is actually significantly smaller than the recommended 3-4 seconds rule with a relatively good margin. It also amounts to ~5 car lengths at that speed, which I rarely see in practice on active highways.
I agree with your instructor. That said, modern cars can stop in /much/ shorter distances than previously, largely thanks to improvements in tire technology.
Nearly every production vehicle available today can brake from 60mph in less than 140ft, more performance oriented vehicles can do so in under 120ft. The standards for “safe stopping distance” used in the US triple this distance to account for slowed reaction time because drivers are not attentive.
Realistically less than doubling these figures is correct for attentive drivers. 60mph is 88 feet per second, and 1 second is plenty of reaction time for an attentive driver, meaning 200-240ft total stopping distance, closer to double the distance the vehicle is capable of rather than triple.
So if the visibility is less than 200-240ft, you should be going slower than 60mph. 240ft isn’t very far in a car though, it’s roughly the distance traveled in 3 seconds at 60mph. There’s certainly some country roads I’d slow down on, but if you’re paying attention you’re well good on most.
For what it’s worth, the average reaction time of a gamer is 150ms, so a full second is plenty of margin of error.
If we care about reducing following distance, we should be lowering speed limits. A car doing 60 needs a much longer safer braking distance than one doing 45.
Also, if you are following at a distance where an AEB activation will cause you to rearend the vehicle in front of you, you are too close to safely follow any vehicle, regardless of whether it has an AEB.
Slower speeds have an outsized effect on safety. Consider that taking 20% off the velocity of the car takes 40% off the kinetic energy of the car, and between 20% and 40% off of the distance it takes for the car to come to a stop.
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)
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.
While it is true that cars regularly are less than a second apart, safety experts give 2 seconds as the minimum safe distance, and 3 is often recommended. You should be the different person who follows their advice, no matter how many people cut in front of you. Better late than dead, and highways are very dangerous.
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.
I would never get closer than a car's length away from the driver in front of me if I was driving 60 mph; why should I act any differently when I'm driving 30 mph? Speed is trivial - the gap must be maintained so drivers have the opportunity to react to cars around them without having to make any drastic changes in speed.
The relative speed difference of 130km/h means you're closing in on the traffic in front of you at 36m/s. That doesn't sound like much but if you consider how long it takes from checking the mirror to changing lanes, that's often just not enough time. Numerous times I've had the situation where I checked the mirror and by the time I've changed lanes, some car almost rear ended me. Relative speed differences like that are just a recipe for disaster and the only way to fix that is to make the fast car go slower.
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.
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.
> You should be able to safely not rear end the car in front of you even if it immediately hits a brick wall and stops completely, […]
I don't think that's the usual expectation. Maintaining absolute braking distance is what railways do, whereas cars commonly operate on relative braking distance plus a safety margin accounting for your reaction time.
Over here, both driving schools and other general road safety education talk about keeping 2 seconds following distance (or half your kph-speed in metres, which corresponds to 1.8 seconds following distance), and traffic engineers commonly give the capacity of a single lane of traffic as 1800 vehicles/hour, which again corresponds to 2 seconds distance (this time as measured vehicle front to vehicle front, instead of tail to front of following vehicle).
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.
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