Ari Bader-Natal

The sidewalker's dilemma

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My bicycle has a flat tire, so I've recently been walking to work. I seem to end up spending much of this time thinking about how to be less late when commuting without a bike. While I always follow the same route when biking to work -- one that balances total distance with hilliness -- I have found no clear best route for commuting by foot. My path changes the most in the Mission, where the ground is flat and the streets are on a grid. I never walk any more than necessary to get there, but I hate the idea of standing still at an intersection while waiting for a light to turn green.

The beauty of walking is that the sidewalk grid is more detailed than the road grid. A standard intersection has two crosswalks on each of the four corners. If you are trying to walk northeast, a North-facing red light isn't a slow-down, since it will be accompanied by an East-facing green light. Since I can't really remember which intersections have lights, I can't really plan ahead too much. So I commute by heuristic.

So this is the sidewalker's dilemma: When arriving at a street corner en route from Point A to Point B, how do you decide whether to turn or to walk? I'll lay out a few variations to make things interesting. Feel free to leave your heuristics (or additional variations) in the comments:

  • Let's start simple: There is a rectangular grid of streets with sidewalks on either side. Your work is n blocks over and m blocks down from your home.
  • Some intersections have traffic lights, but you don't remember which ones. When you arrive at a street corner, you can see the color of the lights at that intersection.
  • You can look a block ahead of you to see if there is a light at that intersection, but you don't know what color the light will be by the time you arrive.
  • Each light is on a different schedule, and the schedules seem to change. Lights have pedestrian crosswalk signals that count down seconds before the light turns. You can see these numbers as you approach the crosswalk. You can muster up the energy to sprint a bit to get to the intersection in time to make the light. But only some maximum number of times during the commute, because you're not really a morning person.
  • You remember a few of the intersections that have lights and a few that do not.
  • There are several coffee shops in the neighborhood. You know where they are, you know you want a coffee, and you don't really care where it's from.
  • You're running too late to walk. You head to the nearest bus stop.

Does this describe your commute? What's your strategy?