DRAFT  Access magazine

 

Why Bicyclists Hate Stop Signs

By Joel Fajans and Melanie Curry

 

                A bike commuter has to consider many things before leaving for work. What’s the fastest route; what’s the easiest route? Even choosing clothing isn’t simple for someone who must plan for ease of movement, safety (dress shoes are slippery, loose fabric can get caught in gears), comfort, exertion level, perspiration, distance, and weather. But the pre-commute subjects pale in comparison to the safety, speed, and energy issues   bicyclists deal with en route. Transportation planners know that incorporating bicycles into the transportation system can help ease traffic congestion by reducing the number of cars; they also know that combining cars and bikes can be tricky. But they don’t always account for the bicyclist’s concerns—matters that don’t occur to the typical planner who is only used to driving cars. Unless they take bicyclists concerns seriously, their planning efforts will do little to increase the numbers of bicycles or help bicyclists and drivers to coexist safely.

 

            Take a simple stop sign. For a car driver, a stop sign is a minor inconvenience, involving shifting a foot from gas to brake pedal, perhaps shifting gears, and, of course, decreasing the car's average speed. These annoyances often cause drivers to choose faster routes without stop signs—leaving the stop-signed roads emptier for cyclists. Consequently streets with many stop signs  are safer for bicycle riders because they have less traffic. Indeed, formal bike routes typically include traffic-calming devices such as barriers and stop signs to discourage car traffic and slow down those cars that remain. However, a route lined with stop signs is not necessarily desirable for cyclists. While car drivers merely shift their feet and sigh at the delay, bicyclists have a whole lot more at stake when they reach a stop sign.

 

Energy Efficiency

            Bicyclists can work only so hard. The average commuting rider is unlikely to produce more energy than the equivalent of about 100 watts of propulsion power—about what it would take to power a reading 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 be 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), cyclists must husband their power. Accelerating from stops is very strenuous, particularly since most cyclists feel a compulsion to regain their former speed quickly. And there's a very awkward moment when the cyclist has to pedal hard to get the bike moving forward with enough speed to avoid falling down, while rapidly upshifting to get back up to speed.

 

            [Energy costs of overcoming inertia are far greater from a standing start than from a rolling start—how much greater? If you don’t know/ don’t have time to find out, can you suggest where I might be able to find out?]  I'm not sure that I understand this question.  The energy required to get to some velocity (ignoring air friction) is proportional to the velocity squared.  This means that if you go to a velocity of 10, you need 100 units of energy if you start from a stop.  But if you roll thru a stop at a speed of 5, you only need 100-25=75 units of energy.

 

            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 cyclist wants to maintain her original 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 all but the most fit cyclists. 

 

            Consider the officially designated bike route on California Street in Berkeley, which 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 2800 feet.

 

            Keeping our exertion constant[1], one of us (JF) one can ride on Sacramento at an average speed of 14.2 miles per hour without straining. At the same level of exertion, our speed falls to 10.9 mph on California if we come to a complete stop at every sign. Thus Sacramento is about thirty percent faster than California. If we increase our exertion to a fairly high level, the average speeds increase to 19 mph on Sacramento and 13.7 mph on California, so Sacramento is 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, in spite of 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, 100watt, cyclist in half.  The extra force required to quickly attain a stable speed on a grade after stopping at a stop sign is particularly grating.

 

One way cyclists conserve their energy at stop signs is to slow down, but not stop.  This is a bit dangerous (though much less dangerous than it seems because the visibility at most intersections is good from a bicycle[2], and assuming that the cyclists has slowed to some rational speed, there's typically plenty of time to stop.)  Of course a sensible cyclist will always slow substantially at a stop sign if there's a car anywhere nearby.  But the car-bike protocol at stop signs is not clear. Drivers (and cyclists, to be fair) are unpredictable. Will drivers take turns with bikes in an orderly way as they do with other cars? Will they start to go, notice the bicyclist, and suddenly stop again to wait, whether the cyclist is stopped or not? Will they roll through the stop without seeing the cyclist? Will they roll through the stop even though they see the cyclist? An experienced cyclist knows anything is possible. For example, if she guesses correctly that the car will wait for her, she’ll want to start pedaling again as quickly as possible, preferably without having slowed much, thereby conserving energy and inertia. Indeed, traffic flow is well served by cyclists not coming to a complete stop as a waiting driver does not have to wait so long for the cyclist to clear the intersection.

 

 

I've deleted to paragraphs here.  They were somewhat redundant, and some of the content has been moved early.

 

Thus, stop signs are a tricky issue for cyclists.  On one hand, they increase cyclist's safety by decreasing the number of cars on a road, and slowing the remaining cars.  On the other hand, they make cyclists work much harder to maintain a reasonable speed.  For a commuter making a choice between a car and a bicycle, the extra exertion can be a serious deterrent.

 

Getting Along

Car drivers say they are confused by the presence of bicycles on the road, and some wish the two-wheelers would just go away. Bicyclists   know that cars cause most of their safety concerns.  . Traffic planners need to find ways to help bikes and cars coexist safely on the road. A good place to begin is by taking the special concerns of bicyclists seriously, and not assume that they will be served by a system designed for cars. Reducing the number of stop signs on designated bike routes   would make bicycle commuting considerably more attractive to potential and current riders. . Perhaps cities should buy bikes for their traffic engineers and require that they ride them to work periodically. There’s probably no better way for them to learn what it’s like to ride a bike in traffic than actually to experience its joys and hazards.

 

Further information on the calculations show here can be found at

http://socrates.berkeley.edu/~fajans/Teaching/bicycles.html



[1] One can keep one's exertion approximately constant by fixing one's heart rate.  For instance, the slower speeds set (14.2 and 10.9mph) were obtained by keeping JF's heart rate at 125 beats per minute (bpm).  This is an easy rate for many cyclists.  The faster speed set (19 and 13.7) required a heart beet of 165bpm.  This high a heartrate  is sufficiently difficult to discourage commuting at this pace.

[2] One rarely mentioned detrimental consequence of the explosion of SUV's, pickups, and vans on our roads is that visibility is significantly impaired.  While this is an issue for car drivers as well, it is particularly significant for cyclists riding in traffic.  Since cyclists can see over the roofs of cars, they can anticipate the flow of traffic many cars upstream.  But cyclists cannot see over the roofs of SUVs, pickups and vans, so their information is dramatically limited.  The problem is compounded by the affection of the owners of this sort of vehicle for tinted glass, which prevents cyclists from seeing through the windows to the traffic ahead.