If you must drive below 40 mph on a limited access highway, what should you use to warn drivers behind you?

Using emergency flashers to warn drivers behind you when you must drive below 40 mph on a limited access highway is an effective way to communicate your slower speed. Emergency flashers, also known as hazard lights, are designed to alert other drivers that you are not operating at normal speeds, thereby providing them with a visual warning that something unusual is occurring. This can help prevent rear-end collisions and encourage drivers behind you to take caution and potentially change lanes to avoid getting too close.

In contrast, brake lights are typically used to indicate that a driver is slowing down or stopping, which may not be sufficient to alert others if they are approaching quickly from behind. Turn signals are used to indicate a change in direction and do not convey that you are traveling significantly slower than the speed limit. Horns are generally used as a warning system for alerting others to immediate dangers rather than indicating slow speed, making them less effective in this situation.