What should the off-peak 85th percentile speed represent?

The off-peak 85th percentile speed represents the speed at or below which 85% of vehicles operate in free flow conditions. This measurement is important because it helps traffic engineers and planners understand how most drivers behave under normal, non-congested conditions. By identifying this speed, they can make informed decisions regarding traffic control measures, road design, and enforcement policies.

This concept is grounded in the recognition that in free-flow traffic conditions, most drivers drive at a speed that feels safe and comfortable, allowing for a reliable depiction of typical driving behavior. The 85th percentile speed serves as a benchmark for setting speed limits and analyzing traffic flow performance. This statistic helps ensure that speed limits reflect the natural flow of traffic, promoting safety while accommodating the majority of drivers.

Choosing a speed that all vehicles must travel or an average speed of all vehicles does not accurately reflect the behavior of the majority of drivers in free-flow conditions. Additionally, a mutually agreed speed by all drivers is not a practical measurement, considering individual preferences and driving styles vary widely. Thus, option C stands out as the correct representation of the off-peak 85th percentile speed.