Newton's first law of motion, in its simplified form, states that a body at rest tends to stay at rest and a body in motion tends to stay in motion.

A car is a body in motion. Therefor it tends to stay in motion. This tendency is called inertia.

A body in motion has a certain momentum. Momentum is mass times velocity.

A fast moving car has velocity (its speed) and mass (its weight).

The faster and heavier a car is, the more momentum it has and the more force it has (inertia) to keep moving and therefor, the harder it is to stop.

Thus if you are tailgating, the faster you are moving, the harder it is to avoid hitting the car in front of you if that car should suddenly slow down.

The rule of thumb is: keep one car length between you and the car in front of you for every ten miles per hour of speed. This means that if you are going fifty miles per hour, keep five car lengths between you and the car you are behind.

I learned in my own Physics classes in high school and college that an object once put in motion will stay in motion unless acted upon by another object. The faster an object is going, the greater the impact that is felt by it upon collision with the other object. It makes total sense that close proximity to the tailgate of another car is not wise at any speed. The driver's reaction time is dramatically shortened and there is less distance to stop if required. In driver's education courses, they teach you to allow a car's length stopping distance for every ten miles an hour you are traveling. So, if you are going 70 mph, you should be seven car lengths behind the car in front of you.

Tailgating involves high speed and keeping a less than safety distance while driving, the purpose may be to overtake a vehicle. The common sense and experience are sufficient to have the awareness of the risk involved. But a little more of dynamics is better to understand the reality of the risk. We know that v^2 -u^2=2as, where v is the velocity of the tailgating vehicle. u has to be zero to bring the vehicle to rest with an acceleration a (actually retardation in this case) and s is the displacement before halt.In fact the equation becomes (as u=0)

v^2=2as. If you multiply by mass both sides we get

mv^2=2mas or

(1/2mv^2)=(ma)*(s)

If the vehicle is in less than safety distance, a high acceleration in oppsite direction is required to stop the vehicle within a short time.Since the vehicle is in less than the safety distance, the chances of accident is more as the high speed requires longer distance to stop. Acceleration is a form of force. So high acceleration is more risky. The reaction time for the driving person is less at high speed. Lastly, V^2 is also a form of energy, see the last equation above. So at higher speed the damage risk is squarely proportional. It is for this reason that a safety distance is defined and it should be squarely proportional to the velocity.

**Tailgating** is the practice of driving on a road too closely behind another vehicle, such as less than the travel distance in two seconds or, equivalently, one vehicle-length for every 8 km/h (5 mph) of the current speed, at 60km/h it is about 33m. Australian rule book describe tailgating as a distance which does not guarantee that stopping to avoid collision is possible.

In some municipalities in the United States and in all the European Union, this action is illegal and punishable by fine.