These two articles appeared in a column by Marilyn Vos Savant about cycling with or against traffic.
“Marilyn: I am a former Missouri State Bicycle and Pedestrian Coordinator, a former licensed road racer, and a current recreational on-road bicyclist. I’d like to add to your response concerning bicycles and traffic.
The reason pedestrians are directed to walk on the left facing traffic is to watch for danger. Most pedestrians, if squeezed by vehicular traffic, may step laterally onto the grass adjacent to the roadway. Bicyclists cannot engage in lateral movement for two reasons. One, the bicycle does not normally roll sideways (it tips instead) and two, the speed of a bicycle would prevent smooth lateral movement.
When the operator of a motorized vehicle approaches a bicyclist from the rear, the motorist has time to notice the bicyclist and plan evasive action. If, however, the two are approaching each other on the same side of the road, the time for evasive action or even thinking is dramatically reduced. Many on-road bicyclists average 15-25 mph on the road, often approaching the posted speed limit of local roads.
Many people wonder why road bicyclists do not ride on sidewalks. The reason is safety. The speed differential between a road bicyclist and a walker is too great to safely share that space. People who walk change direction suddenly or even stop in their tracks, which would cause a bicyclist riding 20 mph to hit them or to run off the sidewalk.
In addition, traffic control devices are constructed to direct those operating a vehicle on the right side of the road. All operators of vehicles must yield to pedestrians in crosswalks.
Many thanks for helping to encourage all to share the road safely!”
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Ned Radich:
“Marilyn: When you wrote about why bicyclists should ride with traffic (and not against it), you neglected to mention the main reason: decreased impact speed in an accident. For example, if you’re riding a bike at 15 mph and you’re struck head-on by a car traveling at 35 mph, you’re exposed to a 50 mph (35 plus 15) impact. But if you’re struck from behind, you get a 20 mph (35 minus 15) impact.
Marilyn responds:
Many readers wrote to express the same idea, but it’s not quite correct. First, the main reason that all types of vehicles (including bikes) travel in the same direction is not decreased impact speed in an accident. It’s the avoidance of accidents in the first place. That’s a basic rule for any kind of traffic.
Then there’s this physics error: In a head-on collision, the sum of the vehicles’ speeds does not equal the force of the impact. This is a common misconception, sometimes propagated by news reports. Say two identical cars are traveling at 50 mph and they collide head-on. Each car sustains a 50 mph impact, not a 100 mph impact.
A cyclist will fare much worse than a motorist in nearly all collisions, of course, but the first one you describe, a head-on collision between a 15 mph bike and a 35 mph car, will not deliver a 50 mph impact to either party.
Why?
Consider the case of the head-on collision between identical cars traveling at 50 mph. They cannot each receive a 100 mph impact—in opposite directions! They must share the combined “100 mph crash.” And as their masses and speeds are the same, they share it equally: Each vehicle receives a 50 mph impact. The effect on each one is roughly equivalent to a crash in which a car runs into a stone wall of such mass that it doesn’t budge. The wall’s share of the impact is minimal; the vehicle bears virtually all of it.
Regardless, if a 20-pound bike collides head-on with a two-ton automobile at a closing speed of 50 mph (15+35), the cyclist’s share will be deadly.
