What speed is used to determine the taper length according to the regulations?

The correct choice is based on the guideline that the taper length for traffic control setups should be determined using the off-peak 85th percentile speed. This measurement reflects a speed at which the majority of drivers feel comfortable traveling under good conditions when traffic is lighter. By using the 85th percentile speed, traffic control designs ensure that taper lengths are adequate for the majority of drivers, promoting safety as well as compliance with posted speed limits.

Using this speed is significant because it takes into account typical driving behaviors during periods of reduced traffic, contrasting with factors like average speed or max speed, which may be influenced by other conditions and could lead to a taper design that does not facilitate smooth traffic flow. The posted speed limit, while important, does not always accurately reflect driver behavior in real-world conditions, especially during off-peak times. Thus, applying the 85th percentile speed allows for a more functional and driver-centric approach in the design of taper lengths.