For an average commercial vehicle being driven at 55 mph on dry pavement, it will take about to bring the vehicle to a stop.

300 feet

Explanation:
Three important factors are involved in bringing a moving vehicle to a stop.

1. Perception distance is the distance a vehicle travels while your brain perceives the need to apply the brakes. An alert driver requires about 0.75 seconds to react. This time accounts for about 60 feet of travel.

2. Reaction distance is the distance traveled before the driver actually presses on the brake pedal. For an alert driver, this also takes about 0.75 seconds. This adds another 60 feet of travel.

3. Braking distance is the distance the vehicle travels before coming to a stop on a dry ground. On a slippery ground (after snow or rain) this distance is longer. On dry ground, the skidding distance is about 170 feet.

This means that an alert driver needs 60+60+170 = 290 feet (approximately 300 feet) to stop.
For this reason, tailgating at high driving speeds is dangerous.