Thanks for putting me in such lofty company. Marshaul is indeed missed. I hope that his career flies.
Apropos the budding engineer Marshaul, I'll mention that a properly engineered speed limit is effective, and is the 85th percentile of the uncontrolled speed observed in the area. The speed limit is not supposed to be the normal and proper speed.
Much as he and I tangled and disagreed on certain non-RKBA social poilcies, I also miss Marshaul's contributions. When he wasn't making a personal attack on me, he frequently had great insights.
The 85th percentile speed is generally the best way to set speed limits on highways, freeways, and major commute routes.
There are, however, other situations where speed limits need to be set via different means including based on the risk of significant injury or death to pedestrians should a collision with a vehicle occur. For example, as shown at
this link the risk of severe injury to a pedestrian hit by a car is 10% if the car is moving at 16 mph. At 23 mph, the risk rises to 25%. At 32 mph, that risk doubles to 50%. And by 58 mph the risk is 90%.
Speed also affects reaction time. Assuming a child runs into the street without warning, a car traveling at 16 mph will cover about 24 feet per second. At 32 mph, obviously, the car covers twice that or 48 feet per second. Additionally, actual stopping time/distance will increase with speed as well. At 16 mph, the car will stop in about 15 feet after the brakes are applied. At 32 mph, that is going to increase to something like 30 feet. As noted
at this link reaction time while driving might well vary between 0.7 seconds (to apply the brakes) to 1.5 seconds.
So total time to stop a car after a child runs into the road is going to be about 50 feet at 16 mph and about 100 feet at 32 mph. And should a car hit a child, the odds of the child being seriously injured jump from 10% (at 16 mph) to 50% at 32 mph.
This report details the limitations of children's ability to judge the dangers posed by cars traveling on the road as well as their inability to properly judge speed and distance.
In other words, when children are present, full responsibility to avoid collision must fall onto the driver of cars as children are simply not capable of making the necessary decisions.
Hence, school zone speed limits of 15 or 20 mph during times when children are present make a lot of sense.
Similarly, residential speed limits of 25 or 30 mph also make good sense regardless of what the 85th percentile speed might be. As we move into commercial areas or major commute routes without homes or school zones, the 85th percentile makes a lot of sense. There are also cases where engineering expertise about what is the safe speed for a section of road may be needed. This is true in cases where a roads risks are not apparent such as being much steeper than is normally perceived, or a turn that is shaper than it appears, or a sharp turn with a negative bank.
Good road design in residential or other populated areas can encourage drivers to drive at a lower, safer speed without relying purely on enforcement. But in many cases, this costs more than building a road on which drivers may want to drive faster than is actually safe for pedestrians or adjacent property owners.
Charles