The 40-20 Rule of traffic safety
Memorize this easy rule so you can bring it up whenever you're talking to someone about traffic calming or pedestrian safety.
Traffic crashes are a menace to society.
America experiences the equivalent of a 9/11 attack every month with no end in sight. Traffic safety is complicated, but it’s obvious enough to say most traffic deaths would be avoided with speed reduction.
There’s no reason to tolerate high speeds in areas with population clusters like cities and the suburbs. Interstate highways are an altogether different story, and those systems should be treated differently. People aren’t turning in and out of driveways on the interstate, and they certainly aren’t walking or bicycling.
In all the ordinary places, here’s why the speed of car traffic matters so much:
If a person walking is struck by a vehicle traveling 40 mph, the pedestrian almost always dies. If a person walking is struck by a vehicle traveling 20 mph, the pedestrian almost always lives.
The “Great American Arterial” is a 6-lane street with a posted speed limit of 45 mph. In light of the 40-20 Rule, consider the absurdity of the modern transportation system. A collision between a car traveling at the legal speed limit and a pedestrian is almost guaranteed to result in death.
If the modernist dream of efficient vehicular travel in urbanized areas really did move people around without much friction (in terms of time and injury), then an argument could be made that the rare, accidental collision was an acceptable price to pay for the greater good. But that’s not the case at all. The quest for efficiency is pursued and human lives are left curbside.
Professionals are aware of the deadly toll, but the same planning philosophy continues to be applied. The root cause is looking for ways to make streets more efficient for motorized traffic.
Here’s the planning & engineering process that makes everything a blur at street level:
Document car speeds through busy intersections during the two busiest hours of a day.
Note that speeds during those two busiest hours are below the posted speed limit.
List options for expanding the street to “increase vehicular efficiency.”
Approve and construct expansion in order to increase
Begin a new study to determine why speeding vehicles are injuring and killing pedestrians.
We improved the corridor by widening the lanes, adding turn lanes, expanding the clear zone, and increasing the turning radii. We made the street more dangerous, just to be safe. —Modern Thought Leaders
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The incredible fact is that American infrastructure in populous areas continues to be built for speedy traffic.
Here are some ways that street networks in cities and suburbs are designed for speed. I’m purposefully using the language you’ll see in traffic studies to help you recognize it.
12-ft wide travel lanes are the norm, even though that width encourages fast driving.
Large turning radii at driveways and intersections are the norm, encouraging faster driving around corners, without pausing for people walking across side-streets.
Large clear zones so that drivers can have unobstructed views, which makes them feel more confident driving faster than they ought.
Multiple turn lanes are constructed to increase vehicle throughput, which makes streets more dangerous for people to walk or ride bicycles.
A transportation system optimized for “vehicular efficiency” can’t simultaneously provide safe passage for non-motorized traffic. The only way out of this cycle is to plan streets for users of all ages and physical abilities.
When you’re behind the wheel, remember the 40-20 rule.
A pedestrian hit at 40 mph will die, but a pedestrian hit at 20 mph will live. I don’t want anyone to get hit by a car, but speed matters in crashes.