Vehicle speed — Illinois Permit Test

Doubling a vehicle’s speed quadruples its stopping distance.

Stopping distance grows with the square of speed, not with speed. That is why a small increase costs so much more than it feels like.

Doubling your speed quadruples your vehicle’s stopping distance.

Speed should be adjusted for the conditions and matched to the flow of traffic, as long as it does not go above the maximum posted speed limit.

Matching the flow is safer than being the odd one out, in either direction. The posted limit is the ceiling on that.

Your speed should be adjusted for the conditions and match the flow of traffic, as long as it does not surpass the maximum posted speed limit.

Large, wide tires with good tread will stop a vehicle faster than small, narrow tires with little tread.

Braking happens in the contact patch between tire and road. Worn tread has less of it and less grip in the wet.

Size of the tires - large, wide tires with good tread will stop a vehicle faster than small, narrow tires with little tread.

A strong tailwind can make it very difficult to stop a vehicle.

Wind from behind pushes the car along while the brakes are fighting it. It is the one weather factor that lengthens stopping distance on a dry road.

Direction of the wind and how fast it is blowing - a strong tailwind can make it very difficult to stop.

What happens to an Illinois vehicle’s stopping distance when its speed doubles?

It becomes four times as long. Stopping distance grows with the square of speed, not with speed. That is why a small increase costs so much more than it feels like. Illinois Rules of the Road 2026, p.84: “Doubling your speed quadruples your vehicle’s stopping distance.”