>Michigan Public Act 85 of 2006 requires speed limits to be set to reasonable speeds for traffic (generally based on the 85th Percentile rule), so as not to artificially incriminate the general population.
While I applaud the intention behind this law, I hope you're overstating Michigan's reliance on the 85th percentile rule. For the uninitiated, "The 85th percentile speed refers to the speed at or below which 85% of vehicles are traveling" [1] as measured by a speed survey of current conditions.
The problem with the 85th percentile rule is that it completely ignores the safety of other road users and the surrounding community, taking drivers perception of safety as fact. I think drivers tend to underestimate the risk that increased speed poses to pedestrians and bicyclists, especially in urban and suburban areas.
Problems exist even on roads that only allow drivers: 'Raising the speed limit to match the 85th percentile speed may lead to higher operating speeds, and hence a higher 85th percentile speed. This generates an undesirable cycle of speed escalation and reduced safety. As a 2016 Insurance Institute for Highway Safety (IIHS) report stated, “The 85th percentile speed is not a stationary point. It is, rather, a moving target that increases when speed limits are raised” (Farmer 2016).'
Rather than relying primarily on the 85th percentile rule, I hope Michigan gives full weight to the additional factors the MUTCD manual mentions, like “(A) road characteristics, shoulder condition, grade, alignment, and sight distance; (B) the pace; (C) roadside development and environment; (D) parking practices and pedestrian activity; and (E) reported crash experience for at least a 12-month period” [1].
If after considering these factors the road is still a speed trap, then the road should be redesigned to align drivers expectations with the posted speed [2], not the other way around.
While I applaud the intention behind this law, I hope you're overstating Michigan's reliance on the 85th percentile rule. For the uninitiated, "The 85th percentile speed refers to the speed at or below which 85% of vehicles are traveling" [1] as measured by a speed survey of current conditions.
The problem with the 85th percentile rule is that it completely ignores the safety of other road users and the surrounding community, taking drivers perception of safety as fact. I think drivers tend to underestimate the risk that increased speed poses to pedestrians and bicyclists, especially in urban and suburban areas.
Problems exist even on roads that only allow drivers: 'Raising the speed limit to match the 85th percentile speed may lead to higher operating speeds, and hence a higher 85th percentile speed. This generates an undesirable cycle of speed escalation and reduced safety. As a 2016 Insurance Institute for Highway Safety (IIHS) report stated, “The 85th percentile speed is not a stationary point. It is, rather, a moving target that increases when speed limits are raised” (Farmer 2016).'
Rather than relying primarily on the 85th percentile rule, I hope Michigan gives full weight to the additional factors the MUTCD manual mentions, like “(A) road characteristics, shoulder condition, grade, alignment, and sight distance; (B) the pace; (C) roadside development and environment; (D) parking practices and pedestrian activity; and (E) reported crash experience for at least a 12-month period” [1].
If after considering these factors the road is still a speed trap, then the road should be redesigned to align drivers expectations with the posted speed [2], not the other way around.
[1] https://www.ntsb.gov/safety/safety-studies/Documents/SS1701.... [2] methods include narrowing lane width, eliminating or reducing shoulders, reducing the number of lanes, increasing road curvature with pedestrian islands, speed bumps, etc