When Is a Stop Sign Not a Stop Sign

Reading this post about a law to allow cyclists to treat stop signs as a yield, something that invariably tends to raise ire among drivers (and some cyclists), reminded me of this post from a while ago — the video embedded there reminds us of the small fact that many drivers (at least in Kansas) already tend to treat “Stop” sign as a yield (at best).

And, in case you haven’t read it, after the jump I’ve posted the classic article (from Access magazine) by physicist Joel Fajans and Melanie Curry, “Why Cyclists Hate Stop Signs.”

(Horn honk to Richard)

Take a simple stop sign. For a car driver, a stop sign is a minor inconvenience, merely requiring the driver to shift his foot from gas pedal to brake, perhaps change gears, and, of course, slow down. These annoyances may induce 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 like barriers, speed bumps, 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 simply sigh at the delay, bicyclists have a whole lot more at stake when they reach a stop sign.

Bicyclists can work only so hard. The average commuting rider is unlikely to producemore 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.

One way cyclists conserve their energy at stop signs is to slow down, but not stop. A cyclist who rolls through a stop at 5 mph needs 25 percent less energy to get back to 10 mph than does a cyclist who comes to a complete stop. Blasting through a stop sign is a bit dangerous (though less dangerous than it seems because visibility at most intersections is good from a bicycle2, and if the cyclist has slowed to some reasonable 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 bicyclists) 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 bicyclist? Will they roll through the stop even though they see the bike? 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 soon as possible, preferably without having slowed much, thereby conserving energy and inertia. Indeed, traffic flow is improved where cyclists do not come to a complete stop, for drivers need not wait long for the bikes to clear the intersection.

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

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. A good place to begin is by taking the special concerns of bicyclists seriously, and not assuming 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 m o re attractive to potential and current riders. Allowing bicyclists to treat stop signs as
yield signs, as some states do, could solve the problems in a different way.

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.

This entry was posted on Tuesday, February 10th, 2009 at 1:54 pm and is filed under Bicycles, Traffic Culture, Traffic Engineering, Traffic Signs. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

10 Responses to “When Is a Stop Sign Not a Stop Sign”

  1. aaron Says:

    How about just getting rid stop signs and making them all yields.

    One thing I realized since getting into mountain biking is that the priciples carry over to driving very well. Brake on straight aways not turns, push a little harder at the bottom of hills, avoid braking, getting up to speed quicker is more efficient… Stops are huge enegy wasters for cars. They should slow down to be albe to stop if necessary, but shouldn’t stop otherwise.

  2. Brad Templeton Says:

    In the past I have proposed a concept of “smart” stop signs, using LEDs and solar power and talking a radio protocol (like 802.11p) to cars and possibly cyclists.

    In such a system (cheap if mass produced) a cyclist would transmit a request for a green light at a stop sign. The stop signs computer would confirm it and show the cyclist a green light on his bike (not on the sign.) However, the stop signs would show a blinking red bright LED of “bicycle coming” to warn motorists to be extra cautious, though they still have to stop at the stop signs like always.

    Cyclists could, given this green signal, proceed through the stop sign at about 10 km/hour, still checking for cars and stopping if need be. If two cyclists both are coming from different directions, they need not stop but will get a signal on their computer telling them the other directions to look out — ideally a map showing the intersection and the location of other bicycles or equipped cars.

    I originally thought of this for cars, so cars with the gear could just blow through 4 way stops in areas where we don’t really want to slow traffic but we have to deal with an intersection. But no reason not to have it apply to bikes at all intersections, even ones where the 4 way stop is to calm car traffic.

  3. Terry Says:

    At particularly dangerous four-way stops, such as when a line of car commuters are taking a residential short cut, I often get off my bike and walk in the crosswalk. Yes it slows everybody down, but there is no question of who has the right of way.

  4. Dave Says:

    I really like the idea of letting cyclists use stop signs as yield signs. Under this law, they still have to stop for an automobile at the intersection if the automobile has right of way, however, it would, as you said, allow them much greater freedom to keep moving and not have to exert the extra effort in stopping and starting again. Under the law, there would also be provisions for making certain stops required for all road users.

    Because cyclists are not enclosed in cars, they can see better and hear better, therefore (if they’re paying attention), they can often see or hear a car or even another cyclist approaching an intersection before it gets there, and therefore are prepared to stop if they need to, or to ride carefully through the intersection.

    I wouldn’t extend the yield to automobiles for a number of reasons. Firstly, I think it’s important in the car-centric world we live in here in the US to sometimes say with policy “we’re not *all* about cars.” By giving bikes or transit priority in at least some situations, I think we make an important statement that we’re at least willing to consider another form of transportation.

    Secondly, if you let automobile drivers treat stop signs as yields, you will increase the speed of traffic on a street, and I would bet, increase the number of automobile collisions at intersections. In any case, stop signs are there to slow automobile traffic, and allowing cars to treat them as yields defeats much of that purpose.

    Thirdly, because people in automobiles are enclosed and have reduced visibility and hearing, I think it’s more important for them to have to come to a complete stop, to be able to assess what is going on around before they just head through the intersection. As a person who regularly drives an automobile, I’m happy to stop at an intersection to make sure I’m not going to cut anyone off by proceeding. It costs me almost no effort, and is much better than rolling through and getting nailed by another car, hitting another car, or hitting a cyclist I didn’t see.

  5. Andrew Says:

    Stop signs are defacto yield signs in a lot of places. Riding a bicycle in Providence, at 4-way stops I do find nearly all drivers nice enough to yield to me so I need not stop, even if it would be my turn to yield were I driving a car. Without eye contact however, I know I had better stop.

    The commenter above thinks stop signs are there to slow automobile traffic. I think this is a bad idea. Drivers are annoyed by superfluous stop signs as exemplified by most 4-way stops. So annoyed, they speed up even more between stops. Average speeds are not reduced, top speeds midblock are higher, and the overall scene of drivers braking, accelerating and being annoyed does not increase safety or “calmness.”

  6. A Says:

    “Drivers are annoyed by superfluous stop signs … So annoyed, they speed up even more between stops. Average speeds are not reduced, top speeds midblock are higher, and the overall scene of drivers braking, accelerating and being annoyed does not increase safety or ‘calmness.’ ”

    100% true.

  7. MikeOnBike Says:

    What exactly do we mean by “yield” at a stop sign?

    Does it mean passing through the stop sign at any speed, as long as cross traffic is not affected?

    Or does it mean slowing to an almost-stop, maybe a couple MPH, but not being required to make a perfectly still stop?

    I think some people are asking that cyclists be allowed to do the former. Or, we could allow all drivers to do the latter.

    If we can tolerate people driving a couple MPH over the speed limit, perhaps we can tolerate people going a couple MPH through a stop sign.

    This would be a rule change or enforcement change that applies to all drivers, not just cyclists.

  8. km Says:

    “Drivers are annoyed by superfluous stop signs … So annoyed, they speed up even more between stops. Average speeds are not reduced, top speeds midblock are higher, and the overall scene of drivers braking, accelerating and being annoyed does not increase safety or ‘calmness.’ ”

    Also, coming to a complete stop and then starting up again is not very Green.

  9. Thom P. Says:

    I think you’re spot on with the yield not stop concept.
    I’ve argued on my own blog about not stopping completely at stop signs.
    Cyclists have 360 degree unobstructed vision, superior hearing capabilities compared to drivers, and are less likely to act like morons at a four way stop simply due to the laws of self-preservation.

    Drivers get irate at Cyclists for rolling through stop signs because they are jealous. Don’t be jealous, get on a bike yourself and we can all roll through stop signs together like one big, happy family.

  10. Ben B Says:

    One thing that most drivers don’t seem to understand when they complain of cyclists not stopping for stop signs is the basic physics of the situation and how it relates to safety. Put simply, a 3000 pound car (passengers included) is 20 times more massive than a 150 pound cyclist (bike included). This means, that a 150 pound cyclist would have to travel at 400 mph to contribute as much energy to a collision as a 3000 pound car would at 20 mph. Similarly, a car at 1 mph is like a bike at 20 mph and a car at 40 mph is like a bike at 800 mph. A cyclist is much closer to a pedestrian in terms of mass and it would be ridiculous to suggest that a pedestrian (or bicyclist) puts passengers of conventional motor vehicles at risk when simply yielding rather than stopping.

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