Friday, October 1

Cyclist Crosses the Road

While I don’t often post about it, I pay close attention to news reports of collisions involving motor vehicles and bikes. In the vast majority of cases, it quickly becomes evident that the collision is remote enough to my riding or driving that it is completely irrelevant. I simply don’t ride on sidewalks, in crosswalks, against traffic or chat on a cell phone while riding, and I rarely ride in locations where a right turning car will squish me. Still, there are sometimes crash elements that relate to things I see or do daily. One such came up a while back in a post Andy, of Carbon Trace, made about a cyclist that was hit crossing a major highway. I imagine it was a road the cyclist had crossed many times before, but this time his luck ran out and it looks as if he was hit by a motorist because he didn’t have quite enough time to get the distance he needed to go.

I opined:
“What I saw on Google streetview was a divided four lane highway, with left turns and a median that the cyclist could have used to confirm his judgment of the highway traffic when he got half way across. It is difficult to judge the traffic on the far side of a four lane road, and using that median could have kept that cyclist safe. He apparently failed to use an advantage (small size) that would NOT have been available to him had he been driving a Ford Ranger.

"THAT is the lesson we as cyclists should absorb unless we wish to become victims ourselves. Suppose the motorist IS distracted, the task remains the same for the cyclist – operate so you get to your destination safely, without close calls. Distracted motorists merely make that task a bit more urgent.
Posted 24 Aug 2010 at 6:33 pm ”

There are ways you, as a cyclist, can reduce your risk of such a collision. One way is to establish a reference mark, such as I describe here for purposes of making crossing the Alliance Gateway Freeway eastbound lanes safer. The reference mark is very handy because there’s lots of traffic along Alliance and I have to cross it every day, so I’m highly motivated to accurately judge the traffic gaps. Another way to make crossing a major street safer is to learn how long it takes to cross big streets, and how quickly oncoming traffic will be upon you.

Table 1. Simple Time Guide to Cross Lanes
I’ll illustrate the basic process. The first element is how long it’ll take to cross a wide street. I have found that I should allow the time shown in Table 1 to get across these things. You may be significantly quicker or slower, but my table works for me on my commute. These are 12 foot wide lanes, but you won’t get across narrower lanes a whole lot quicker. Maybe a second or two, but you’ll have to work out just how quick YOU cross a street.

The second element is how quickly an oncoming motor vehicle will intersect your path. Conveniently, many streets include reference marks such as telephone or light poles that can be used as a rough guide. Telephone pole spacing varies, but 100 to 125 feet is typical. For safety’s sake, we’ll assume 100 feet.

Table 2 shows how many phone poles away a car needs to be to give the appropriate time window if the poles are 100 feet apart. In each case, I have rounded partial seconds DOWN to the nearest full second. You will note that if you want a really simple rule of thumb, just figure that an oncoming car will travel one telephone pole’s distance each second. That’ll be safe for city streets, and will be a reasonable guide even for getting across higher speed roads. If your street has periodic driveways, you can create an equivalent table simply by measuring (rough measurement will do) how far apart they are and knowing that:

 time in sec = 0.681818 X (number of features) X (distance apart features are in ft) / (car speed in mph)

Table 2. How Fast the Cars are Coming at You
If you want to cross a road on your bike safely, you need to arrive past where the car is traveling before it gets there. It’s as simple as that. If it takes 6 seconds to cross a seven lane road, you are in big trouble if the oncoming motorist occupies the same space 6 seconds hence. You want to pass that space far enough in front of the motorist that he/she feels no need to take evasive action. Evasive action by a motorist with the right of way can end badly for the cyclist attempting to cross the road. You want happy, content motorists that do not come into contact with you or your bike and that don’t have to think too hard to make that objective into a reality.

If I apply this to crossing a seven lane road such as Davis in North Richland Hills, I know that it’ll take me 5 to 6 seconds to get across the far lane. If a motorist is coming along at a clip of 40mph, I know that if he is 4 telephone poles away, it’s not going to be safe to try to cross in front of him. Because I like to leave extra margin, I’d probably want a bit extra if I were going to cross – the driver might be going faster, or I might miss a shift. As a result, I'd probably only cross if the car was 6 poles away. This'd give me clearance even if the driver were bombing along at 60mph. On the other hand, if the traffic is two lanes away from me, I can cross if the oncoming motorist is 4 telephone poles away because it takes me only 3 seconds to cross that lane.

All this math has an added side benefit. Counting how long it takes to cross various streets builds the traffic senses and seeing how fast cars are traveling also sharpens the sensibilities. Of course, all the finer points will vary for you, just as they do at various locations along my commute route. KNOW your motorists! It is one reason why, when I fell on Monday, no motorists became involved with me afterwards.

If you like things REALLY SIMPLE, just make sure that oncoming traffic is more telephone poles away than you have lanes to cross. No math needed. Just see that oncoming motorist is far enough away that you can cross without turning into a pancake. This is not about absolute calculation precision, but about using available visual references to guard against a misjudgment by yourself. You'll get better with very little practice, and it won't slow you down on your commute by a single second or cost you a penny. Actually, it'll probably get you to your destination a smidge quicker - without close calls.


John Romeo Alpha said...

This unit of distance measure is known as the Fahrnuff, named after the Frenchman Pierre d'Fahrnuff. The Fahrnuff may be applied in single lane passing situations when you have to judge the distance of any oncoming traffic in the passing lane, as well as crossing a street. The distance to the vehicle being assessed is = 1 Fahrnuff if it is farnuff away that it won't be involved with you when you do what you a intend to do. Some of us build in a conservative safety factor into our Fahrnuff calculations, while others are plainly not very good at estimating a Fahrnuff. Hopefully your post helps.

Chuck Davis said...

If more riders wood take the time to read the Fahrnuff Papers and to understand the basic priciple and factor into their day to day and regular rides they can reduce the luck running out element

Those that can read the original in French pick it up toot sweet; those who struggle avec some of the poor translations out there and just blow it off tend to get hurt more often

twister said...

Should I now, begin to carry a calculator with me? =)
I think it should be assumed that people aren't going to see you and they aren't going to be slowing down for you either. If they do see you, it's likely, that in the times in which we find ourselves, you'll find the motorist speeding up in attempt to make you stop, so you'll have to amend your calculations on the fly.
Reading the occasional post from motorists on Facebook about bicyclist's on the roadways, I'm taken aback at the tone of aggrieved and aggressive comments made by said motorists. It doesn't help that helmet hair Perry vetoed the passing of some laws that would strengthen the bicyclist's position on the books.

Steve A said...

Twister, I will do a post on the presumption that people aren't going to see me, and another about what motorists SAY versus what they DO. In the case of the first, remember Murphy. In the case of the second - as in the case of blacks during Jim Crow, the minority are kept in line by threats from the majority rather than by overt actions. If threats don't work, the police can sometimes be enlisted to help out since they're motorists with guns. Actual serious violence is rare, especially if witnesses may be present.

HINDSIGHT! If I had this post to do over again, "Fahrnuff" would be in the title somehow!

Chandra said...

Very useful post!

RANTWICK said...

You, my friend, are definitely an engineer, as was my Dad; I have leanings, but didn't go that way myself. Outstanding. Well done.

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