According to the Manual on Uniform Traffic Control Devices, when a speed limit is posted, it should be within 5 mph of what speed?

When a speed limit is posted, it should be set within 5 mph of the 85th percentile speed of free-flowing traffic. This approach is rooted in the principles of traffic safety and roadway design, as the 85th percentile speed reflects the speed at which 85% of drivers are traveling at or below under free-flowing conditions.

Setting speed limits in accordance with the 85th percentile speed helps ensure that the majority of drivers are compliant, which can lead to a more uniform flow of traffic. This method is based on the understanding that drivers tend to react to their peers and drive at speeds they feel are reasonable and safe for the specific roadway conditions. If the speed limit is set too far below this threshold, compliance rates tend to drop and can create hazardous conditions due to varying speeds among drivers.

Thus, aligning speed limits with the 85th percentile speed promotes both safety and smoother traffic flow, making it a best practice in traffic engineering and management as per the guidelines of the Manual on Uniform Traffic Control Devices.