The Geometry of Safe Cycling

When I come to an intersection, in my mind’s eye, I draw a bird’s eye view of my path and the potential paths of all the vehicles in my range of vision. In other words, I visualize a mental map of potential collision points in a plan view, like a GPS display, but showing not just where to go, but also where the potential threats are coming from. All of this happens semi-subconsciously in the blink of an eye. I’ve done it for many years, and I admit this sounds a little weird, and I don’t know how it got started, but I think it helps keep me safer on the road.

I’m only guessing, but I suspect this habit of visualizing a “collision map”, if you will, may be a result of the fact that I’m a graphic designer that works (and consequently thinks) in two dimensions all day. Among other things, I create a fair amount of technical graphics and maps. The mapping in particular seems closely related to this unusual habit. (My wife is always confounded with my keen sense of direction and ability to read maps, while I’m continually confounded by her ability to remember precisely what someone said in a conversation three weeks ago… LOL. I think this demonstrates something about the left brain versus right brain paradox.) But I’ve terribly digressed, so back to the point…

I believe one of the most important things we can do to stay safe on the road is to anticipate the actions of our fellow road users. That’s why I use a rear view mirror; if I see a car drifting onto the shoulder or into the bike lane I have an extra split-second to take evasive measures (this has saved my life at least once, maybe twice). The same holds true for left and right hooks (the deadliest of all one of the more common accident types); anticipating that a car might hook you by visualizing its potential path buys a split-second that may be just enough to avoid getting hit.

I’m not suggesting anyone make a conscious effort to draw a virtual map in their mind every time they come to an intersection (that’s far too distracting unless it’s something that comes naturally), but I am suggesting it behooves all cyclists to get in the habit of anticipating where other road users are headed. Doing so is arguably the best defense against a collision.

9 Responses to “The Geometry of Safe Cycling”

  • Tom says:

    I have been thinking of getting a mirror. I heard the ones that attach to the helment are difficult to get used to. What do you use?

  • Alan says:

    Hi Tom,

    I use the Bell rear view mirror made specifically for my Bell Metro helmet. Frankly, it’s not the best. In my opinion, the most effective for either helmet or eyeglass mount is the old “Take-A-Look” mirror. They take a little getting used to, but in short order they become second nature. It’s kind of funny, but I’ve become so accustomed to having a mirror available that I often find myself looking up to use a mirror when I’m walking along in a parking lot; of course, since I don’t have my helmet on, there’s no mirror there… LOL.


  • Ian says:

    I too have been using a helmet attached mirror for the last few years. I find it a wonderful accessory and even though a little smaller than I would like,it is a great stress reliever when riding in traffic. Usually the first indication of approaching traffic is audible, then a quick check of my helmet mirror gives me a heads up confirmation of the overtaking circumstances, which is much easier to assess than trying to look over my arthritic neck and shoulder while trying to maintain a straight line.
    Since advising my wife that when I am on the bike she should expect my return using a calendar rather than a clock (grin! grin!), I’m thereby able to pick “the road less traveled” knowing she can always contact me on the cell phone.
    Of course, being retired, the clock is not quite as important as it was, so I’m a very privileged guy to be able to pick and choose my route, its timing, and duration.
    Alan, I really like your blog and never miss reading it. I do hope you can do some of your blogging while riding the rails or bus etc on your laptop.

    I have acquired the Walmart special 6 speed folding bike to take to our summer place. I have tried it out here in the city a little and it seems that it should be good for the short trips into the village to get groceries etc.

  • Croupier says:

    As someone who has been hit by several cars (and survived, 100% intact) I have a very simple piece of advice that might be useful if you cannot avoid being hit by a car all together (but DO try really hard not to get hit). I actually learned this while playing baseball: If you know you’re about to get hit, do everything you can to get your feet off the ground.
    In baseball you have to jump accomplish this, on a bicycle you have to resist the urge to stop and put your feet down. Getting your legs caught or pinched beneath a car because you tried to brace yourself for an impact is far worse than what will happen to you if you let your bike take a hit and your roll your body onto the hood of a slow moving vehicle.
    Of course, this assumes the vehicle is slow moving.
    Also, wear a helmet, kids.

  • david says:

    Interesting piece. I work with maps frequently, as well, and often find myself behaving and thinking similarly while riding. I take issue, however, with one of your points.

    I’m curious what data you’re looking at in determining that left/right hooks are “the deadliest of all accident types.” In my experience working with crash data, I’ve seen otherwise.

    For starters, the bicycle fatality dataset as a whole, particularly at state and local levels, is relatively small to begin with, so when you start pulling out a particular “crash type” for analysis you end up with VERY small sample sizes (maybe 10 of a particular crash type over a decade)…and, well, the stats on sample and populations sizes that small tend not to be of the most reliable sort. You can aggregate severity levels to get a larger population (grouping “fatalities” with “incapacitating injuries”, for example), but even then the population size is relatively small, and EVEN then, I’m pretty sure that right/left hooks don’t represent the largest share (maybe if you add them both together, but to do so would be somewhat disingenuous, as the turning movements, direction of travel, and force and angle of impact are quite different).

    The reason I bring this up is because certain crash types – especially right hooks – seem to get a lot of attention for their perceived level of severity, when in reality, the numbers just don’t back it up. While right hooks are among the most FREQUENT of crash types, there’s little evidence (yet tons of media attention) supporting the statement that they are among the most dangerous.

  • Alan says:


    Your point is well taken. The statistics vary so greatly depending upon the source, I should have worded that phrase differently (I’ll make a correction).


  • torrilin says:

    I don’t draw a mental map. Instead, I read the “body language” of the other users’ lane positioning. Car drivers are very consistent in the visual language that they use… a right turn is indicated by pulling close to the curb. A left means they’re hugging the center line. Someone going straight has a very balanced stance. An impatient person will creep forward, and a patient one chooses a stopping point and *stops*. And there are certain sorts of fidgeting that indicate the driver is confused and is about to do something dangerous.

    Cyclists are not as consistent. Experienced and VC leaning ones are much easier to read, but novices or bike ninja types can be very hard to follow… they don’t *want* to give consistent signals. (and the novices spend a lot of time confused as to what is a good signal) I’m slowly learning to spot some of the signals, but they’re awfully subtle. No wonder car drivers can get frustrated!

    When I use “carlike” position signals, it seems like drivers can predict my movements better. The right and left tire tracks that cars make on the roadway are very helpful for judging what position will give them the clearest indicator. It’s not a substitute for turn signals, but every little bit helps when you’re shooting for a crash free life.

  • John Links says:

    Hurray! I do something similar, more like watching a TV broadcast of me & traffic from a helicopter… sort of. A constant overhead picture of all moving objects and their relative velocities. I developed this & several other techniques as a motorcyclist. Watch the front wheels of cars – you’ll know what they’re going to do before they do. Also, pay attention to sounds – it’s like spider sense.

  • Ernie Greenwald says:

    Regarding mirrors:

    I’ve tried various mirrors over many years of cycling. For riding in traffic, I like the “Mountain Mirrycle.” I have one on a BikeE-AT and another on a Trek 720 that I use for local shopping and errands, and two on a Bacchetta Corsa that I ride for longer distances. These are quite different bicycles, but the “Mirrycle” works well on all of them. Use on the Bacchetta requires a one-inch extension of the mirror-base arm, but stability of the mirror does not suffer. The extension is a $10 factory option. If you have a good local hardware store, you may find the necessary spacer and cap screw for considerably less.

    Regarding traffic safety:

    I’ve not been hit by a car in over sixty years of riding. I think the most important rule I enforce on myself is: “NEVER pass a slow-moving vehicle on the right.” Once, near Santa Barbara, I was behind a cyclist who decided to pass a large trash-compacting garbgage truck. He began to pass on the right, in the middle of a block. With no warning, the truck turned right, into an inconspicuous driveway. All four of the right-rear wheels of the truck went over the bike. The driver continued on–oblivious. Happily, the cyclist was thrown clear. Not one single piece of the bicycle that was undamaged! Even the chain was broken.

    I saw a similar–but more predictable–accident when a young lady tried to pass a city bus on the right side. She was struck when the bus moved toward the curb to pick up passengers.

    So I don’t bother to judge if passing on the right might be safe in any particular situation. I just don’t do it. Depending on traffic, I’ll either stay behind, or pass on the left. Passing on the right isn’t worth the time I might save. I’m not through getting old yet.

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