Space ahead
The basic rule for following distance is one second per ten feet of vehicle length at speeds up to forty miles per hour, plus one additional second at speeds above forty. A sixty-foot tractor-trailer at fifty-five miles per hour needs at least seven seconds of following distance. Measure by picking a fixed object, starting your count when the vehicle ahead passes it, and verifying that you do not pass the same object before reaching the count.
Space behind
You cannot fully control space behind you, but you can avoid making it worse. If a vehicle is tailgating, do not speed up — that often makes the tailgater more aggressive. Move right when safe and let the tailgater pass. Avoid sudden braking that might force the vehicle behind you into a collision.
Space to the sides
Stay out of other drivers' blind spots, give buses and other large vehicles wide turning room, and never drive next to another large vehicle for any longer than necessary to safely pass it. The right side of the truck has the largest blind spot — a fact that motorcyclists and small-car drivers regularly underestimate, and that you will be tested on.
Space overhead and below
Many bridges and underpasses post clearances; if your loaded vehicle exceeds the posted clearance, do not enter regardless of what your GPS tells you. The clearance below your vehicle matters too — railroad crossings, washboard shoulders, and steep driveway approaches can all bottom out a low-clearance trailer in a way that damages the vehicle and blocks the road.
For drivers preparing for Space and Following Distance, additional context — including federal manual excerpts, employer hiring practices, and DOT medical guidance — is widely available from industry resources. Continue reading on a recommended industry resource for further detail. Always cross-check anything you read with the current edition of your state CDL manual, since enforcement guidance is updated periodically.