Here's why:

* Kinetic Energy and Impact Force: The force of impact is directly related to the kinetic energy of the vehicle. Kinetic energy is calculated by the formula: KE = 1/2 * mass * velocity^2.

* Doubling Speed, Quadrupling Energy: When you double the speed of a vehicle, the kinetic energy increases by a factor of four (2^2 = 4).

* Impact Force: Since impact force is directly related to kinetic energy, doubling the speed means the impact force increases by four times.

Therefore, at 20 mph, the impact force of your car is four times greater than at 10 mph, not just "about four times".