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What are the principles involved in this bicycle scenario?

  1. Feb 19, 2009 #1


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    What are the formal principles of math and physics that describe this scenario?

    Most local laws require bicyclists to ride with (i.e. in the same direction as) traffic. The rationale behind this is that as a car and a bike are traveling in the same direction the car has more time to react to the bike before overtaking it (when compared to the bike and car traveling towards each other).

    So how would you describe this in terms of the formal scientific principles involved? Please use very specific terms, and tell me how exactly they describe the above.

  2. jcsd
  3. Feb 19, 2009 #2


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    Staff: Mentor

    Welcome to the PF. Why are you asking this? Is it coursework, for some legal reason, personal curiosity, or what? Could you maybe take a cut at the description you are looking for, and we can tell you if you have it right, or if it needs some tuning up?

    Knowing the context of your question would help us respond better.
  4. Feb 19, 2009 #3


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    This is for an educational research project in bike safety education. Currently, riding with traffic is seen by bike safety educators as a high-priority safety practice. However, it is generally taught as a rule to follow without providing an underlying rationale.

    I'm going to compare the knowledge of experienced vs. inexperienced cyclists regarding safety practices such as riding with traffic. My hunch is that experienced cyclists have a deeper understanding of the rationale behind such practices, even if they can't verbalize a formal rule, and that bike safety education should describe the rationale to inexperienced cyclists.

    I need to be able to communicate such rules in a formal manner. So for example I need to be able to say something like "The principle of linear mass inertia tells us that if a car is traveling at 50 mph and a bicycle is traveling at 20 mph, given the same level of visibility the driver of the car will have x more seconds to react before meeting the bicycle if the two are traveling in the same direction than towards each other."

    I realize there may not be a physical law or model that describes this as neatly as I would like. So please feel free to give me a math formula or whatever else describes this.

    Any help or suggestions would be much appreciated!
  5. Feb 19, 2009 #4


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    In the UK Highway Code, Rule 2, for pedestrians, "If there is no pavement (sidewalk) keep to the right-hand side of the road so that you can see oncoming traffic" (except on sharp right-hand bends) …

    I think bicycles used to be regarded as pedestrians, and were also advised to face oncoming traffic …

    since the only advantage of seeing oncoming traffic is so that you can jump into the ditch if necessary (after all, the traffic can see you anyway), and that isn't really practical on a bicycle, it's difficult to see any advantage at all.

    Riding with the traffic means that the traffic can see you for longer before reaching you, and has a lower relative speed and therefore both a shorter stopping distance and less impact (change in your momentum) if it hits you.

    Of course, this is on land … different rules apply when cycling underwater.
  6. Feb 19, 2009 #5


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    Ah ha! Momentum! That's the kind of thing I'm looking for. Can you identify some of the other physics concepts that come into play and how?
  7. Feb 19, 2009 #6


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    BTW, another advantage of bikes riding in the same direction as the (generally faster) cars, is that they are more visible. Especially when you wear brightly colored jerseys. Because the bicyclist is leaned forward in an aero position usually, there is much more exposed jersey area facing to the back, as compared to the front, where the head/helmet and arms block a lot of the jersey color.

    Not exactly a Physics angle on the question, but a very real effect with respect to safety.
  8. Feb 19, 2009 #7


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    The advantage of riding against traffic is that both the bicycle rider and the car drivers can see each other, which is why it's recommended for pedestrians (so why not bicyclist below a certain speed?). The disadvantage is the rate of closure is faster.

    The other concern about riding against traffic would be going across an intersection at the same time a car going the same way makes a left turn towards the bicyclist. In areas where it's legal to turn right on a red light, approaching an intersection would be an issue because the car driver making a right turn isn't expecting something moving fast against traffic.

    Sidewalks would be the safest place (assuming bicyclist would slow when near pedestrians), but it's not allowed in the USA.
  9. Feb 20, 2009 #8
    In safety courses on motorcycle riding, there is an important principle called "target fixation". This is when a nervous driver or rider sees a potential danger ahead, in this case he may fixate his gaze upon the hazard and unwittenly drive DIRECTLY into it. Instead, we are taught to: acknowledge the hazard, then immediately look where it is best to steer in order to avoid the hazard. That is, look towards the solution instead of continually onto the problem.

    OK, back to your bike. I would venture to say that a car driver may be overwhelmed in seeing a bicycle approaching almost directly at him with a vary rapid rate of approach (the sum of their speeds). If the car driver makes the mistake of target fixation, then the cyclist runs a credible risk of impact.

    In contrast, a cyclist riding with the traffic would not be perceived by a car driver as a direct hazard. The driver would recognize that it is he who is approaching the slower bike, and not the bike being the one approaching the car. As such, the driver would not consider the cyclist as being a hazard. This allows the driver to remain calm, and thus maintain a better level of self control.

    As you acknowledged, the impact the cyclist would experience is the function of his relative speed with that of the car. Further still, the cyclist would sustain injury in relation to his direction of impact; ie, is his rear impacted or his skull. As an example, for a car travelling 50 kph and a cyclist going 20 kph:
    1) Frontal impact would have the skull being impacted at 70 kph.
    2) Rear impact would have the butt being impacted at 30 kph.

    I would venture to say, the first scenario would often be fatal, while the later may result only in injury.
    Last edited: Feb 20, 2009
  10. Feb 20, 2009 #9
    I don't think so. I think impact energy is more significant. The mass of the rider times the closing speed squared.

    And reaction time and braking time as a function of closing speed.

    But then again, what happened to "unsafe in traffic at any speed?" I've got these guys parading around on their bikes around my place. They seem to like seeing two directional traffic go frantic on the blind turns, aggrivating a problem where half the drivers are unskilled upon roads that are anything but straight. What would it take for them to play somewhere else?
    Last edited: Feb 20, 2009
  11. Feb 20, 2009 #10


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    "Impact energy" is another good term - thanks!

    Don't know what to say about cyclists "playing" somewhere else. The first automobile accident ever recorded in the US was a car vs. bicycle, and there has been conflict ever since. Just please think about the variety of benefits cycling vs. driving provides to society in terms of decreased traffic, decreased pollution, increased health, etc., and try to accomodate those who choose to do it.
  12. Feb 20, 2009 #11


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    Thanks for the responses thus far - it's been a big help. Below are concepts I've pulled out of the above replies. If you can think of any more please add them. Now I'd like to see if I can have you rank these concepts in order of their relatedness to the scenario. In other words, if "momentum" is more directly related to the scenario than "motion," simply move "momentum" above "motion." I'm trying to whittle down to a list of say the 8 concepts most directly related to the scenario.

    Reaction time




    Rate of Closure

    Sum of Speeds

    Force of impact

    Impact energy

    Stopping distance

    Impact speed

    Rate of approach

    Direction of impact
  13. Feb 20, 2009 #12
    Automobiles and bicycles are an uneven mixture of horsepower, kinetic energy and impact shielding.
  14. Feb 20, 2009 #13


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    Hey, no fair! You left out my visibility thingy :cry: I was up all night thinking that one up!
  15. Jul 15, 2009 #14
    Think about it: if a bicycle is riding towards a car at 30mph, and the car riding at the bike at 40mph, they're coming at each other at 70mph! whereas, if the bike is moving the same direction as the car at 30mph, and the car at 40mph. the car is coming up on the bike now only at 10mph, giving the motorist MUCH more time to react and reducing the possibility of complete fatality from a crash.
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