Drivers don’t follow the “two-second rule”

Flow (q) is the number of people vehicles past a point per unit time. Capacity is the maximum flow. This depends on the road, the vehicle, and the driver. If we were all race car drivers, at the same spacing between vehicles, we could get a lot more vehicles past a point. Alternatively, if we drove a lot closer together at the same speed, we could also increase throughput. However there are reasons we don’t do this. We have to consider human reaction times. When taking driver’s education, you may have learned the two-second rule, in good weather follow the driver ahead of you with at least a two-second headway. If everyone did this, we could get 1800 vehicles per hour per lane past a point.

Recall there are 3600 seconds in an hour, dividing by a 2 seconds per vehicle gives 1800 vehicles per hour 3600/2 = 1800. In fact, limited access freeways in good weather during peak periods have a much higher throughput, sometimes observed as high as 2600 vehicles per hour per lane, though more typically between 2000 and 2200, indicating people are ignoring the two-second rule, and are following more closely. The reason for the 2 second rule is that if the driver ahead of you slams on the brakes, you have some amount of mental processing (Perception-Reaction) time to see the brakelights, tell your foot to move from the accelerator to the brakes, and push hard yourself.

Robots won’t follow the “two-second rule”

Human drivers are instructed to follow the car ahead of with a two-second following distance. We expect autonomous (self-driving) vehicles could follow even more closely, and thus increase throughput on roads that they would produce. A world of connected vehicles might be closer still since information about acceleration and braking is broadcast as it is happening from downstream cars. and does not require a brake-light-detection system before engaging, thus allowing robo-cars to safely follow closely.

Robots also won’t need lanes to be as wide, since they will be able to drive with much greater precision. Both closer following and being closer laterally will significantly increase the number of vehicles that can use a given amount of road space, and may allow us to shrink our roads.