Rules of the Open Road

| July 11, 2006

I am of an age (53) to remember when travel in the United States primarily involved roads that had, at most, two lanes in each direction (and that only if you were lucky). Interestingly enough, once you get out of major urban areas, it still does. Long cross-country stretches of the US Interstate Highway system are still just two lanes in each direction, as I can attest; our car, only a year old, has over 26,000 miles on it, mostly from open-road travel ranging from Washington DC in the east to San Diego in the west, and most of it spent on I-70, I-80, I-15, and their various offshoots.

I can also attest that many of the people driving those interstates have no clue about how to drive on a road that has just two lanes in each direction. Various reasons suggest themselves. They are too young to have experienced cross-country travel on US 101, US 395, or the still-legendary US 66. They mostly drive in urban areas, where freeways tend to have 3 or more lanes in each direction. They haven’t thought about how traffic flow works, or they just don’t care.

On the other hand, I grew up knowing the single most important rule of cross-country driving: Keep Right Except to Pass. I knew this rule before I ever sat behind the wheel of a car myself. And it’s clear that some people still know and appreciate this rule: my adopted state of Colorado has recently adoped a law restricting the left lane for passing use on freeways where the speed limit is 65 MPH or greater (PDF brochure here). Other states have previously enacted similar laws.

But there’s more to smart open-road driving than that. During the many hours of cross-country driving that Sandra and I have done in the last year, I have been working out as simple a set of rules as I can for two-lane (i.e., two lanes each way) driving. One of my areas of professional interest is complexity theory, particularly the emergence of intelligent, adaptive behavior from a small set of carefully-chosen rules, and with that perspective, I have tried to come up with a simple set of rules to maximize traffic flow on two-lane-each-way roads. While I have not yet done a computer simulation to test my ideas, I do have a candidate list comprising just two major rules, with some sub-rule elaborations. The two major rules are:

  • Rule #1: Avoid causing other drivers to slow down.
  • Rule #2: Don’t be ignorant.

Rule #1 — Avoid causing other drivers to slow down — includes the classic “Keep right except to pass” rule, but it also includes several other behaviors:

  • Pass only to go faster. If you’re not planning to drive faster than the car in front of you, then don’t pass.
  • Don’t cut in front of passing cars. If you are in the right lane and a car is rapidly approaching in the left lane, don’t cut over to the left lane yourself; instead, let the car pass, and then move over. Obvious exception: if you can do so without forcing the passing car to slow down, i.e., the passing car is some distance back and/or is not approaching very quickly.
  • Speed up to pass. I know, sounds obvious, but you would be surprised how often a car moves over to the left lane to pass, then takes a minute or longer to do so. Which leads to the next subrule…
  • Pass only if you can do so quickly. I have respect and admiration for the driving skills and habits of truckers in every aspect but this one. It is especially egregious on uphill climbs, when one semi pulls out to pass another and takes what feels like an eternity to do so, blocking all traffic behind in the meantime.
  • Don’t cut in front of the car you just passed. Make sure that you can see the entire front of the passed car in your rearview mirror before moving.
  • Speed up on moving back into the right lane. Time and again I’ve seen one car pass another, move in front of the passed car, and then slow down to the same speed (or even slower!) as the passed car. This is really just a restatement of the Pass only to go faster subrule above, but it’s worth emphasizing.
  • Move left if possible to make room for merging traffic. When a car is merging onto the freeway, it will likely be moving more slowly than you. If you are approaching it as it is coming up to speed, move left to give it room. On the other hand…
  • If you are merging onto the freeway, accelerate up to speed quickly. Floor the accelerator pedal if you have to, but get up to speed as fast as you can.
  • Don’t speed up when someone wants to pass you. If your speed has dropped and someone wants to pass you, let them. Once they have gone by, if you want to pass them in turn, feel free to do so. But speeding up in the right lane when someone is trying to pass you in the left lane is just ignorant. And speaking of which….

Rule #2 — Don’t be ignorant — encompasses both the literal and the colloquial sense of that phrase. In other words:

  • Be aware of the driving conditions. This, of course, is Drivers Ed 101, but there appear to be a lot of drivers out there who don’t pay attention to the cars around them, the weather conditions, the road conditions, the current curvature and slope of the road, and so on.
  • Don’t be rude, stupid, hostile, and so on. Look, folks, we’re all traveling at speeds of 60 to 90+ MPH with friends and loved ones in metal contraptions weighing anywhere from several hundred pounds to many tons. This is not the time to show off, play dominance games, express rage, and so on.

At some point, I may code up a simulation to test out these rules, particularly the subrules for Rule #1. But a lifetime of empirical observations convince me that these rules allow traffic to flow quickly and safely on freeways with just two lanes in each direction.

A final note: when traffic density increases, the specific subrules of Rule #1 given above become less applicable, though Rule #1 remains a good general principle. Traffic speed is inherently asymmetrical — that is, a car or group of cars can slow down from speed Y to speed X much more quickly than they can speed back up from speed X to speed Y. This is the reason for Rule #1 in the first place. However, when traffic density increases to a point where the right lane is, in effect, always full, the left lane begins to fill up with regular traffic, rather than just passing cars.

Those are my observations; I’m interested in hearing what others have to say. ..bruce..

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About the Author ()

Webster is Principal and Founder at Bruce F. Webster & Associates, as well as an Adjunct Professor of Computer Science at Brigham Young University. He works with organizations to help them with troubled or failed information technology (IT) projects. He has also worked in several dozen legal cases as a consultant and as a testifying expert, both in the United States and Japan. He can be reached at, or you can follow him on Twitter as @bfwebster.

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