Driving and Physics : the laws of the road that cannot be broken

   There are two sets of laws involved with driving a car. The first set is made from the human-made laws which are a set of etiquette laws of how people can share the roads and passageways while driving 700+ kilogram vehicles. The other, underlying, laws are associated with the laws of physics. The first set can make interacting with other vehicles, and their drivers, more predictable -- and generate income for the various cities in which you drive. The second set -- unlike the situation for the Coyote or Bugs Bunny in Looney Tunes -- cannot be broken and determine what will happen when your car interacts with other physical objects.

   The first topic that I'll mention is that of relative velocities. If you are driving 30 mph (or 50 kph, if you prefer) and you run into a wall, there is 30 mph (squared times half your mass) amount of force with which you hit the wall. This is called kinetic energy. If you run into something coming towards you (such as another car) at 30 mph, the amount of energy is quadrupled and it is as if you hit a wall at 60 mph. (Note that hitting a wall at 60 mph does more than four times the amount of damage as 30 mph.)

   If two cars are going in the same direction then the collision energy is subtracted. If you are driving at 60 mph along a road and someone driving at 65 mph bumps into you then it is the same as if they drove into a wall at 5 mph. Not very noticeable unless that bump makes you lose control and you use your 60 mph kinetic energy to run into a tree.

   This is the principle behind merging. The idea is that you drive your car such that you are driving at approximately the same speed as other cars by the time you leave the on-ramp to the highway. You speed up a little and safely merge ahead of a car. You slow down a little and safely merge behind a car (preferable). If you are going 30 mph while all the other cars are going 60 mph it makes it much difficult for you, and for all the other cars, to merge safely.

   This leads to the next topic -- "tailgating". This is where you are traveling at a speed such that it is not possible to stop without colliding into the car ahead of you if they abruptly stop. My old traffic books indicated one car length per 10 mph -- so, around 90 feet for travelling at 60 mph. I routinely see people driving with a single car length between cars while driving 60 mph. This situation is very dangerous for two reasons -- if the car ahead abruptly stops, you have converted a 5 mph bump into a 60 mph crash into a wall. The second reason is that it makes it very difficult to merge. The merging car cannot safely go ahead, or behind, other cars on the highway if there is no room.

   Tailgating is directly involved with another law of Physics called inertia. This is Isaac Newton's "First Law of Motion". It says that if something is at rest it will want to stay at rest (difficult to start moving) and if it is in motion it will be difficult to stop and will continue at the same speed and direction unless outside forces change it.

   So, let's apply this law of inertia to tailgating. Stopping distance involves the factors of "reaction time" and physical stopping time. Reaction time varies between people. In general, women have better reaction times than men and, in general, younger people have better reaction times than older people. However, reaction time can never be zero as it takes time for the outside signal to reach your eye (you see a brake light) and your brain to process the signal to start a reaction (stepping on the brake).

   This is why it is impossible to stop if you are tailgating someone and they abruptly stop no matter how good are your reflexes. IF you both started stopping at exactly the same time AND both had the same tires and same braking systems on your cars THEN tailgating could be safe. Those factors are not true.

   This leads into the final topic for this blog. Driving in weather. Weather affects a lot of factors. It can make visibility easier or harder -- which affects reaction time. It can make the traction of the tires on the road better or worse -- which affects physical stopping time. Results from relative velocities remain the same but the outside aspects which makes a difference in how, when, and why have changed.

   So, if you drive on a snowy, icy road you have increased reaction time, physical stopping time and the principles of inertia make it more difficult for you to either turn or stop without the car wanting to continue in the same direction and speed.

   In the end, the laws of physics will exist and they won't change so merge properly, don't tailgate, and allow for changes in road conditions for judging safe driving.

[formula for kinetic energy corrected due to the sharp eye of Dave Albay-Yenney]