## Comment: Re:Is this at least user-selectable? (Score 1) 470

I just did some back of the envelope calculating on this. In a typical car with decent performance, you can accelerate at about 4 meters/sec sq (about 0-60 in a little under 7 sec). Working in meters because it's a lot easier: imagine this, an intersection with good visibility. At 40 mph approach speed, the earliest you can tell that the other driver is going to blow through the intersection (at right angles to you) will be 2 seconds. Before that he's still got time to stop. But let's say you're suspicious of the other driver so at 2 seconds out you are instantly ready to take action (and not taking action will result in the front corner of your car making the initial collision with the front corner of his car). Your cars are Ford Focus length, 4.5 meters long. You are both doing 40 mph (18 meters/sec).

So where are you at time = 2 if you decide to accelerate? The reference point is the leading edge of your car. The distance you travel will be determined by in this case the function d(t)=18t + 2t^2. At 2 seconds no part of your car must be between 36 meters and 37.8 meters from the position where you decided to hit the gas (so the leading edge of your car must not be at the position 36m to the position 37.8m + length of your car, which is 4.5m, so 42.3m). If you hit the gas the leading edge of your car would be at 44m, so you only just miss and you need to have a high performance car to do that (Focus ST or Focus RS). If you're in a more normal car, or an older car that's a little bit worn out, and have a 0-60 time of 9 seconds (3 m/s squared), the formula would be 18t + 1.5t^2, and the leading edge of your car will be at 42m, in other words the other vehicle will clip the rear of your vehicle and you will now have the additional speed to some how get rid of during the ensuing crash.

What about braking? A typical car will decelerate at 8.2m/s^2 if you slam on the brakes. ( http://www.michigan.gov/docume... ) So the distance formula for braking will be 18t - 4.1t^2. If you were to slam on the brakes, at the critical time the leading edge of your car would be 19.6m from your starting point - you'd miss the collision by a very comfortable 16.4 meters. Even if it were lashing with rain, and your braking performance were halved, you would miss the collision by almost 10 meters.

The conclusion here is that the margins are much much tighter (in the best case, you only get away with it by just over a meter) if you try to accelerate than if you try to brake (where you miss the collision in the worst case by better than 9m). Acceleration in reality would probably be worse than calculated if you're in an automatic transmission car because you won't really start accelerating much until the transmission sorts itself out. In a manual you'll only be better off if at the decision point you're already in the ideal gear for accelerating. Acceleration may be a valid path to take if you are in a Bugatti Veryron or a Lamborghini Countach or on a motorcycle, but even so the margins are going to be much more comfortable if you mash the brakes instead (given a super car has very good brakes, and a performance motorbike has sticky tires and very good brakes). And if the collision does occur, if you've braked there's a great deal less energy in the system so the outcome is likely to be much less severe.