Left Turners and the “Decision Line”

You assess multiple risks while riding. Will that pedestrians cross the street? Will that car merge into my lane? What emergency route do I take if the car in front of me slams on his brakes? All perfectly valid concerns, but all of these hazards can be avoided fairly easily when compared to the following question: Will that car turn left in front of me? This question is of the utmost importance when I am riding in the city. “Damn it!” I scold myself, if I don’t notice a car that is preparing to make a left turn. They should not be the primary concern that blocks out all other variables but when I ride left turners are on my high priority list.

I do this every day, on every ride yet I have never read or heard this talked about. Before every left turner I create an imaginary line I call the “Decision Line.” Noticing a left turner brings about two options: Do I break hard or rock on the throttle to avoid collision? When I am passing through an intersection that has a left turner waiting I assess my speed and determine an imaginary line that splits these two decisions in half. If the car turns into my path and I am in front of this imaginary line that means I can brake sufficiently to avoid the car. If the car turns into my path and I am beyond the imaginary line I must give it all the throttle I can. This line marks the point that, once crossed, even the harshest of braking won’t stop you in time to prevent colliding with the car.

Note: It’s not as simple as either stopping or going. Swearing enters into the picture as well, but for the purposes of this discussion I will limit it to these two aspects.

Visualizing this line prevent you from having to decide whether to brake or accelerate. The obvious question arises of “How do I determine this line?” It will be different for every rider and bike. Some riders can brake more efficiently than others and some bikes have better or worse brake capabilities. Determining this line is better attuned when one has an accurate sense of how much distance it takes to bring your bike to a stop. You can develop this sense on the road or in an empty parking lot. When I am commuting and getting off on an exit ramp I practice emergency braking. I make sure there are no vehicles behind me and when I am at 60MPH I coming to a stop as fast as I can. I like practicing this way because I am not limited to the same old parking lot and am exposed to different road conditions.

The follow diagram describes a common scenario for motorcyclists and the “decision line” provides less hesitation or guesswork from the rider should a car turn in front of you.


(Mind my crude drawing of a motorcycle when viewed from above)

Aside from reacting to a car turning in front of you, I find preventing the situation from happening in the first place to be the first course of action. These are a couple things I do:

* Ensure I am in the left turner’s line of sight. Often this means moving to the left side of the lane, or slowing down behind trucks that might obstruct their view of you.
* Flashing my brights multiple times. This has become automatic for me now. I flash my brights at every left turner I encounter. Day or night. Maybe I’m blinding people at night, but I don’t really care and I don’t see this making the situation any worse.
* Perform attention grabbing swerving well before you enter an intersection. Be sure that the swerving does not continue into or close to the intersection as it might get in the way of evasive maneuvers you might have to perform. I rarely do this except in intersections that are prone to collisions. They might be thinking “What the hell is that guy doing?” but at least they notice you exist.

I’ve been riding for three years and just about 40,000 miles now and I have never had someone turn left in front of me. Thankfully I haven’t had to put this decision line to the test, but nonetheless, I still prepare for it. I think the preventative methods I listed above have gone a long way to prevent left turning catastrophes. There have been countless times where drivers are inching forward as I approach, but I adjust my position to remain in sight, flash my brights or wiggle around to get noticed.

What does everyone else thing about this? Is this an effective method? Do you do anything similar? Are there any other techniques we can use to help out decision making process in situations where fractions of a second matter?

  • Windchaser

    I tend to agree with your comments. Another factor I find all too often necessary is what is behind me and how close are they. Safe riding is keeping a balance between what is (or could be) in front of you but also what is behind you. Got hit from behind once in a senario you described but it wasn’t a left turner but a large piece of earthmoving equipment that wanted to share my lane. I slowed to let him clear me but the truck behind me did’nt!

  • http://www.atlasrider.com Atlas

    @Windchaser

    Sorry to hear about the accident. I always check my mirrors whenever I am braking. Whether it be an aggressive brake, or relaxed. It is a habit I’ve formed by now that comes automatically.

    The priority of my attention shifts based on the conditions. If I’m braking it’s behind me, if I’m accelerating it’s ahead of me, and if it’s at an intersection then it’s almost everywhere.

  • Bucky

    Good writeup.

    I have a high beam headlight modulator (Comagination VisiPath sS75H). This seems to reduce the tendency for people to pull out in front of me. Modulates during daylight, is steady in darkness. The Ninja 650R low beam headlight remains illuminated when the high beam is on, so there is both a steady and a modulating headlight during the day.

    The 650R mirrors provide only a very poor view to the rear without squirming and moving arms out of the way — not things I want to do when trying to avoid an accident. I plan on trying a bar-end mirror to augment my rear view.

    Also, probably should have the right hand poised over the front brake lever in such situations. Saves a fraction of a second, if braking is required.

  • http://www.atlasrider.com Atlas

    I’ve been thinking about getting a headlight modulator. How do you like yours? How was the installation?

    I have to tuck in my elbows slightly to see behind me, but I haven’t found it to be a a big enough deal to get bar end mirrors. About a year ago I remember people over at the ninja650r.com forums talking about getting mirror extensions with some success. Check them out if you are looking for an alternative to bar end mirrors.

    Good point about covering the front brake. I always have my front brake covered with two fingers while in traffic, forgot to mention that.

  • Nutrix

    Interesting comment on the Headlight flash, here in the UK that would be a ‘courtesy’ signal for the car ahead to turn 😯

    Think this is common in several other countries, worth checking before hitting the roads in Italy 💡

  • http://www.atlasrider.com Atlas

    @Nutrix

    I never heard of the headlight flash being interpreted the way. Good to know though, thanks.

  • prtsimmons

    Good advice. It reminds of the ‘point of no return’ I learned about in Young Drivers of Canada (15 years ago, but it’s still good advice). We were told to determine the ‘point of no return’ for yellow lights – i.e., identifying a point to determine whether to brake for the yellow or drive through the intersection. Obviously, it’s more appropriate for new drivers, but I think your ‘decision line’ is a similar, good idea for motorcyclists.

    I always cover my front brake with my fingers as I go through intersections, and I try to see where the potential problem drivers are looking. I think I will be trying your ‘decision line’ technique in the future.

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