Space Management and Speed Control
Space Management and Speed Control is the domain where the exam expects you to calculate, not just recite. Following distance for a CMV is measured in seconds, not car lengths.
Following distance
- Under 40 mph: one second for every 10 feet of vehicle length.
- Above 40 mph: add one more second.
- A 60-foot combination at 55 mph needs 7 seconds; a 70-foot double at 50 mph needs 8.
Space around you
- When stopped, leave enough room to see the rear tires of the vehicle ahead and to steer around it.
- If someone tailgates, increase your own following distance so you can slow gradually instead of brake-checking.
- Slow down before curves and in rain; posted speeds are for cars, not loaded trucks.
Worked example: You drive a 70-foot double at 50 mph. Base following time is 7 seconds (one per 10 feet), plus 1 second because you are over 40 mph, for 8 seconds. At 50 mph you cover about 73 feet per second, so 8 seconds is roughly 587 feet - about two-tenths of a mile.
Traps: using the "one car length per 10 mph" rule; keeping the posted speed in rain or fog; braking hard in a curve; leaving no escape gap at a stoplight; forgetting that heavy loads need longer stopping distances.