When driving at 60 mph, how does stopping distance compare to driving at 30 mph?

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Stopping distance is made up of two parts: the distance you travel while you react to a hazard, and the distance your car travels while you brake to a stop. The reaction distance increases directly with speed, because you cover more ground in the same amount of time. The braking distance increases roughly with the square of speed, since you have to dissipate more kinetic energy at higher speeds even with the same braking force.

Doubling the speed from 30 mph to 60 mph means the reaction distance doubles, and the braking distance grows about fourfold. If those two parts are similar in size at the lower speed, the total stopping distance becomes about three times as long: 2x for reaction plus 4x for braking equals roughly 3x of the original total.

For example, if at 30 mph the reaction and braking distances are each about 50 feet (total 100 feet), at 60 mph the reaction distance is about 100 feet and the braking distance about 200 feet (total about 300 feet). So the stopping distance is roughly three times longer. Real numbers vary with reaction time and braking effectiveness, but the idea that stopping distance increases significantly when speed increases holds.