Tucson Bike Lawyer has some great commentary regarding an article on the physics behind the trouble cyclists have with stop signs. Here is a bit from the original article and some commentary from TBL below…and comments from me below that.

Bicyclists can work only so hard. The average commuting rider is unlikely to produce more than 100 watts of propulsion power, or about what it takes to power a re a d i n g lamp. At 100 watts, the average cyclist can travel about 12.5 miles per hour on the level. When necessary, a serious cyclist can generate far more power than that (up to perhaps 500 watts for a racing cyclist, equivalent to the amount used by a stove burner on low). But even if a commuter cyclist could produce more than a 100 watts, she is unlikely to do so because this would force her to sweat heavily, which is a problem for any cyclist without a place to shower at work.

With only 100 watts’ worth (compared to 100,000 watts generated by a 150-horsepower car engine), bicyclists must husband their power. Accelerating from stops is strenuous, particularly since most cyclists feel a compulsion to regain their former speed quickly. They also have to pedal hard to get the bike moving forward fast enough to avoid falling down while rapidly upshifting to get back up to speed.

For example, on a street with a stop sign every 300 feet, calculations predict that the average speed of a 150-pound rider putting out 100 watts of power will diminish by about forty percent. If the bicyclist wants to maintain her average speed of 12.5 mph while still coming to a complete stop at each sign, she has to increase her output power to almost 500 watts. This is well beyond the ability of all but the most fit cyclists.

We decided to test these calculations on an officially designated bike route in Berkeley, California Street. The street is about 2.25 miles long and nearly flat (average grade 0.5 percent). Traffic is very light, which is nice for cyclists. But California Street has 21 stop signs and a traffic light. More than two-thirds of the route’s 31 intersections require a stop—that’s one every 530 feet. A parallel route, Sacramento Street, runs one block west of California Street. Sacramento has four lanes of traffic and can be very busy, especially during rush hours. With cars parked along both sides of the street, Sacramento has little room for cyclists. But it has only eight traffic lights along the section parallel to California’s bike route, and no stop signs. Since, on average, only half the lights will be red, there ’s only one stop every 2,800 feet.

One of us (Joel Fajans) found that keeping exertion constant1, he could ride on Sacramento at an average speed of 14.2 miles per hour without straining. At the same level of exertion, his speed fell to 10.9 mph on California if he stopped completely at every sign. Thus Sacramento was about 30 percent faster than California. By increasing his exertion to a fairly high level, his average speeds increased to 19 mph on Sacramento and 13.7 mph on California, so Sacramento was then 39 percent faster. While a drop of a few miles per hour may not seem like much to a car driver, think of it this way: the equivalent in a car would be a drop from 60 to 45 mph. Because the extra effort required on California is so frustrating, both physically and psychologically, many cyclists prefer Sacramento to California, despite safety concerns. They ride California, the official bike route, only when traffic on Sacramento gets too scary.

These problems are compounded at uphill intersections. Even grades too small to be noticed by car drivers and pedestrians slow cyclists substantially. For example, a rise of just three feet in a hundred will cut the speed of a 150-pound, 100-watt cyclist in half. The extra force required to attain a stable speed quickly on a grade after stopping at a stop sign is particularly grating.

TBL’s comments:

Exactly! Other things to keep in mind:

1. Bicyclists tend to enter every intersection with caution, whether they have a stop sign or not, because cars often do not see us. We slow down, and sometimes stop, far more often than cars do at intersections where we already have the right of way. We are far more alert than drivers (see No. 2).

2. Bicyclists have tremendous built-in incentives to be cautious at intersections. We won’t suffer a fender-bender if hit–we will very possibly die or be severely injured.

3. Bicyclists can see and hear far better than drivers because we don’t have blind spots and we are not encased in glass and steel. Also, simply being on the bicycle, out in the open air exercising our bodies makes us more alert and more mentally present in our surroundings.

4. A bicyclist’s eyes and ears are some of the first things to enter an intersection. A driver may not be able to see an intersection clearly until the front of his vehicle is six or eight feet into the intersection. In such a case, by the time a driver can see, he is already risking getting hit.

5. A driver is constrained by the road and by the size of his vehicle as to where he can be when he approaches an intersection. A cyclist can maneuver her bicycle sideways in the lane to see more clearly and hear more clearly, often long before she reaches the intersection.

6. As the above article points out, stops signs are a tremendous disincentive to cyclists, causing them to choose dangerous arterials that only disrupt drivers more and endanger the bicyclists.

7. The rear-view mirror and two front corner blind-spots in every car are in precisely the worst place for determining whether an intersection is safe to drive through. This alone argues strongly for making otherwise safe intersections require a stop for a car. But bicycles do not have these problems.

We all know that some bicyclists, usually young, inexperienced ones, sometimes fly through stop signs. The proposed law does not legalize that behavior. It requires bicyclists to enter intersections with appropriate caution.

What the proposed law does is recognize that there are important, fundamental, physical differences between a bicycle and a car, and some of our traffic laws need to accommodate and recognize those differences. Some already do, of course — bicycles are not required to have brake lights or turn signals, for example, even though some marginal increase in safety would probably accrue if they were. Bicyclists in Arizona also cannot lawfully be ticketed for driving too slowly, as cars can. Bicyclists are allowed to ride on the shoulder, but cars are not.

We already have different laws for bikes and cars where it makes sense to do so. If we want to encourage people to ride bikes, and if we want to encourage them to ride in safe places, then Rep. Patterson is right: this is a “fair, common-sense bill.”

This isn’t about preferencial treatment for cyclists, it’s about fair, safe accomodation for a growing number of people taking to the street on two wheels. For cyclists, I’ve always thought stop signs should be regarded as yield signs and stop lights should be treated as stop signs. Until the laws are changed, it’s best to look behind for cops before doing anything currently illegal. As more people ride, the laws will change.

The best thing any cyclist can do to remain safe is to pretend you’re invisible. Cars can’t and won’t see you. Don’t assume they do. It is very easy to shoot through traffic like a ghost.

This entry was posted on Wednesday, February 18th, 2009 at 11:42 am and is filed under bikes!. You can subscribe via RSS 2.0 feed to this post's comments. You can comment below, or link to this permanent URL from your own site. Your comments will be moderated but will appear as soon as humanly possible.