Walk Signals

A walking sign in Madrid decorated by the Yipi Yipi Yeah street art collective.

Our puzzle this week was suggested by Seth Cohen, who teaches astronomy and physics at St. Paul’s School in New Hampshire. We last saw Seth a year ago, when he illuminated us with Bulbs and More Bulbs. Let’s now join him for a race through town. Here’s —

Let’s say that you are in a square city with a perfectly square grid of streets which is 3 blocks by 3 blocks (a very small city). Think of a tic-tac-toe board, where each line is a street. You are standing in the SW corner (i.e., in the SW corner of the SW block). Your goal is to get to the NE corner, which you do by walking N/S/E/W along a path of your choosing.

It takes 1 minute to walk a city block. Once you reach an intersection, you have to wait for the walk signal to cross. Each walk signal fires once per minute, but the various walk signals in the city are not synchronized in any way, even within a single intersection (don’t mind the inconvenience this might cause for cars).

Assume that a walk signal stays on for only one second before turning off, and when a walk signal fires you leap instantaneously across the street as the walk signal is shutting off. Each individual walk signal turns green at the same time each minute. Assume you make it across the street. And you can’t cross an intersection diagonally.

What’s the best strategy/path to get you to the NE corner in the least amount of time?

Check reader comments on Friday for a solution and recap by Seth Cohen.

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