Following Distance Behind a Large Truck
You are on a three-lane interstate posted at 65 mph, traveling in the middle lane at 65. Ahead of you is a large semi-trailer truck, also in the middle lane, going about 62 mph. You have been following it at roughly one second of gap. The truck's rear is tall and flat, and you cannot see its mirrors or the road far ahead past it. Traffic in the left lane is moving faster, around 70 mph, and is fairly steady. Your exit is about two miles ahead on the right. It begins to drizzle, and the pavement takes on a slight sheen.
1. Evaluate your current following distance and what it costs you.
Read the worked answer
One second at 65 mph is far too close - that is roughly 95 feet, well under any safe following distance and especially short for a large truck. A common guideline is at least 3 seconds in good conditions, and more when following a large vehicle or in rain.
Following a tall truck this closely also blocks your forward view: you cannot see past it to spot slowing traffic, debris, or the road ahead, and the truck driver cannot see you in the mirrors. That combination removes your ability to anticipate and makes a rear-end collision likely if the truck brakes suddenly.
2. What should you change, and by how much, given the drizzle?
Read the worked answer
Back off to a much larger gap - at minimum 4 seconds in the drizzle, and consider more given the truck's size and your blocked view. At 65 mph, 4 seconds is about 380 feet.
Increasing the gap does three things: it restores your sight line around the truck, it gives you room to brake smoothly on the wet sheen, and it puts you where the truck driver can see you. The drizzle raises stopping distance, so the extra space is not optional.
3. You need to exit in two miles. How do you plan the lane change with the truck and the faster left lane?
Read the worked answer
Plan early rather than cutting across at the last moment. First, increase your following distance so you have a clear view and room to maneuver. Then, when it is safe, move to the right lane well before the exit - do not try to pass the truck on the right at the last second, since trucks have large blind spots on both sides and may move right.
If you choose to pass, do it on the left with a clear gap in the faster traffic, signal, and complete the pass without lingering alongside the truck. But since your exit is right, the simpler and safer plan is to get into the right lane early, adjust speed to the lane, and take the exit with time to spare rather than threading between the truck and faster traffic at the last mile.