Speed Control and Following Distance
What this domain covers
Speed control and following distance questions test the basic speed law: you must never drive faster than is safe for current conditions, even below the posted limit. Missouri sets absolute limits, but rain, fog, ice, traffic, and curves can make the posted number too fast.
Core rules
On Missouri rural interstates the maximum is generally 70 mph; urban interstates and other roads have lower posted limits. Unless otherwise posted, school zones are 25 mph when children are present, and work-zone limits apply when posted. The maximum is for ideal conditions. Use the three-second rule: pick a fixed object ahead, and when the vehicle ahead passes it, count three seconds before you reach it. Add at least one second for rain, night, or heavy traffic, and more for ice or fog. Stopping distance rises with speed and roughly with the square of speed, so doubling speed greatly increases stopping distance. Increase following distance behind motorcycles, trucks, and vehicles you cannot see around.
Worked example
You are behind a truck at 55 mph on a wet road. At 55 mph, three seconds covers about 242 feet; rain reduces traction and visibility, so use four or more seconds. The reasoning: the posted limit does not shorten the truck's stopping distance or yours, and a truck's spray can hide a stopped vehicle. Back off until you can see the truck's mirrors and have a four-second gap.
Traps
Counting "one-thousand-one" too fast; using the posted limit as a target in fog; tailgating to prevent others from merging; and following a large truck so closely that you cannot see its mirrors or the road ahead. Also remember that a lower speed limit does not require you to drive exactly at it if conditions are worse.