Light Truck, Wet Road, Four-Second Gap
You are on I-5 in a five-axle tanker that has just finished delivering 6,800 gallons. About 150 gallons of liquid remains in the unbaffled barrel, and the truck now scales 32,000 lb instead of 78,000 lb loaded. Rain has been falling for an hour and standing water sits in the wheel paths. You are at 55 mph, the posted truck limit, holding what you believe is a four-second gap to the car ahead. The car brakes hard. A yellow sign reads 'Rough Road Next 3 Miles,' and traffic is heavy in the right lane.
1. Estimate your real stopping distance on this wet road, including reaction time, and compare it with your four-second gap.
Read the worked answer
At 55 mph you travel about 81 feet every second. Allow 1.5 seconds of perception and reaction, roughly 121 feet, before the brakes do anything.
On wet asphalt assume a friction coefficient near 0.35. Braking distance is speed squared divided by twice the friction coefficient times gravity: 81 x 81 = 6,561; 2 x 0.35 x 32.2 = 22.5; 6,561 / 22.5 is about 290 feet. Add the reaction distance and you need roughly 410 feet to stop.
A four-second gap at 55 mph is 4 x 81, about 325 feet. You are short by roughly 85 feet, and that is the best case with perfect brakes and full attention. The four-second rule is a starting point for cars on dry pavement, not a stopping guarantee for a tanker in the rain.
2. You are 46,000 lb lighter than when loaded. Does that shorten your stop enough to make the short gap acceptable?
Read the worked answer
Not reliably. Once the tires are at the limit of grip, the friction force available scales with the weight on them, so deceleration is roughly the same whether the truck is heavy or light. Mass cancels out of the skid equation.
What does change is bad. The brake system was balanced and tested for a 78,000-lb vehicle, so on a light trailer the brakes can lock wheels and flat-spot tires, and a locked wheel gives you no steering. The 150 gallons of residual liquid, maybe 1,200 lb, is free to run the length of an unbaffled barrel and gives a shove forward right at the end of the stop, which is when you least want it. A light trailer also bounces on rough pavement and skips. Do not trade weight for following distance.
3. The car ahead brakes hard. Walk through your actions in order, and explain what the Rough Road sign adds to the decision.
Read the worked answer
Release the throttle, apply the brakes firmly and progressively, keep the wheel straight, and let anti-lock do its job: brake and steer rather than pump. Check the mirrors for the traffic behind and pick an out before you need one, but do not swerve into a heavy left lane. If you must leave the road, do it with the wheel straight and at the lowest speed you can manage.
The Rough Road sign matters because braking over broken pavement makes wheels skip and lock, and because you should have reduced speed before the rough section rather than during it. Standing water raises hydroplaning risk quickly at 55 mph, and a hydroplaning tire cannot brake at all. The right answer is to slow before the hazard, not to manage it at speed.