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Commentary: Bike Physics
Art Olson | Avenida Primavera

Obeying the laws -- cars, bicycles, apples.
Photo illustration Art Olson

With Del Mar’s streetscape plan moving forward, it is important to consider how we use our streets, paths and sidewalks, and the impacts that these uses have. Most agree that bicycling is a wonderful alternative mode of transportation. It is kind to the environment and healthy for those who ride. However, there are clashes over how bicycles, cars and pedestrians interact. Much of it comes down to the question “what laws should bicyclists follow?”. Currently there are two kinds of laws that pertain – traffic and physics. While they don’t always mesh, you can’t change the laws of physics. The traffic laws mostly see cars and bicycles similarly, but from a physics point of view they are very different.

In physics, momentum is the product of mass times velocity. When comparing an automobile traveling at 30 miles per hour to a bicyclist moving at 15 miles per hour the difference in momentum is roughly 1000 times greater. This can be most easily seen by the disparity in damage when these two modes of transportation collide, but it also has implications when they don’t. In the context of momentum, a bicycle is more akin to a pedestrian than an automobile. So, based on the laws of physics, should our traffic laws distinguish between cars and bikes? The most obvious approach for bicycle safety is to separate cars and bikes as much as possible – bike lanes do this with painted stripes and occasional signs; bike paths do this with by providing a physically separate path of travel – a much preferred and safer solution. Del Mar is beginning to explore this second option on the south end of Camino Del Mar.

Beyond that, however, the bicycle is held to the same traffic laws as the automobile. Many drivers complain about, and many riders are cited for, bikes rolling through stop signs. Again, momentum underlies another physical concept called inertia – as Newton described it “a body in motion tends to stay in motion…” and bicyclists are averse to coming to a complete stop, since “a body at rest tends to stay at rest.” This means that unlike a car, for which the driver requires only the tap on the accelerator, a bicyclist must use significant force to move from a full stop to restore forward motion. Of course, if there are pedestrians or cars crossing the path at a stop sign, the bicyclist must stop, but if not, rolling cautiously forward is momentum conserving and not an unreasonable move. In some jurisdictions, this so called “Idaho stop” has been adopted for bicycles, where stop signs for cars are treated as yield signs for bicycles. In Idaho, since its implementation, this has reduced bicycle accidents by 14%. (for a good video explanation see: www.cupolamedia.com/bicycle-media-the-idaho-stop).
Del Mar’s streetscape plan should not only beautify our main thoroughfare, it should promote both sustainability and safety by encouraging and enabling a welcoming environment for bicycle travel. Understanding the laws of physics and adapting the traffic patterns and laws for bicycles would go a long way toward this end.


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