5.4 Space Below and the Space Needed to Cross or Enter Traffic

Space Below and Space for Crossing or Entering Traffic

Space management is usually taught as a set of cushions around the vehicle — ahead, behind, to the sides, and overhead. Two of the remaining dimensions are the ones that strand trucks and cause the most spectacular grade-crossing collisions: the space underneath the vehicle, and the space you need in traffic before you commit a long, slow-accelerating vehicle across a lane.

1. Space Below: The Clearance Drivers Forget

Many drivers never think about the space under their vehicle, and on a fully loaded trailer that space can be very small. Suspension deflects under load, so the same trailer that cleared a hump empty may drag when it is grossed out.

Where Vehicles Get Hung Up

• Dirt roads and unpaved yards. Ruts, soft shoulders, and washboard surfaces reduce effective clearance and can bottom out the frame, the fuel tanks, or the landing gear.
• Drainage channels and dips across a road. A shallow drainage swale that a pickup crosses without noticing can catch the ends of a long vehicle — the rear of a trailer or the front bumper of a tractor — as the wheelbase bridges the low point. Cross such depressions carefully, at an angle where practical, and slowly enough that the suspension is not loaded and unloaded violently.
• Railroad crossings. This is the highest-consequence case. Humped or elevated crossings can trap trailers with low underneath clearance. Never take a chance on getting hung up halfway across the tracks.

High-Profile and Low-Clearance Crossings

Road authorities post a low ground clearance grade crossing warning sign where the crossing profile is steep enough to catch a long or low vehicle. Treat that sign as a routing decision, not a suggestion: if your rig is a lowboy, a car hauler, a stepdeck, a moving van with a drop frame, or any single-axle trailer with a long overhang, find another route. The types of equipment most at risk are:

If you do get hung up on tracks, the response is not mechanical, it is immediate: get out of the vehicle and away from the tracks, then call the emergency notification number posted at the crossing or 911, giving the crossing's DOT identification number and location. A stuck trailer is replaceable.

2. Space Needed to Cross or Enter Traffic

Be aware of the size and weight of your vehicle whenever you cross or enter traffic. Three facts drive every judgment here:

1. You need a much larger gap than a car does. A commercial vehicle accelerates slowly and is far longer, so it occupies the conflict zone for much more time.
2. Acceleration varies with the load. The same tractor that merges briskly when bobtailing will crawl at 80,000 pounds. Allow more room if your vehicle is heavily loaded — and remember that the grade of the road you are entering changes the answer again.
3. Before you start across a road, make sure you can get all the way across before traffic reaches you. Stopping halfway across a lane with a 70-foot combination leaves your trailer exposed to traffic that cannot stop in time.

Judging the Gap in Practice

A useful discipline is to think in seconds of exposure, not in car lengths. Ask how long your vehicle will occupy the lane you are crossing, then require a gap comfortably longer than that.

The Most Common Error

Drivers judge the gap from the tractor, because that is what they can see. The vehicle that matters is the rear of the trailer — it is still in the conflict zone long after the cab has cleared. When in doubt, wait. There is no penalty for a gap you decline, and no recovery from a combination stalled broadside in a travel lane.