I was interviewed last week for this article by Peter Korn of the Portland Tribune. This blog post is inspried by my attempt to describe the signal timing in downtown without any technical terms to a member of the media. I was sure I struggled mightily and the first time I said quarter cycle offsets, I expected his eyes to roll, but we drew up a sketch discussed the math and related that to his experience. The sketch looked an awful lot like what I had previously written for the Signal Timing Manual back in 2008 while working for the FHWA. Here's the text:
A similar method of manual coordination timing can be applied to downtown grid networks. This method has been deployed in downtown Portland, Oregon by separating intersections into a quarter cycle offset pattern. The block spacing in downtown Portland is fairly uniform and relatively short (280 feet) and the grid is a one-way network. Each subsequent intersection is offset by a quarter of the cycle length, which is selected to progress traffic in both directions. The result is a progression speed that is dependent upon the cycle length. This approach establishes a relationship in both directions of the grid and permits progression between each intersection in each direction based on the speed that is a result of the selected cycle length and the block spacing. As shown in Figure 6-18 cross coordination throughout the grid is achieved using the quarter cycle offset method. This approach can be adjusted to account for turning movements within the grid and subtle modifications to the distribution of green time.
In downtown Portland, the p.m. peak hour cycle length is 60 seconds, which results in a 15 second time difference between subsequent intersections. To travel the 280 feet in 15 seconds, one must travel (280'/15sec) or 18.67 feet per second or 13 miles per hour. The lower the cycle length, the faster the travel speed. Thus, Downtown Portland has progression in multiple directions at a slow speed which is especially good for buses that are accelerating from a stop, cars that can drive through at a consistent speed and come to a quick stop if someone in front of them pulls out of a driveway unexpectedly, and reasonable for people travelling on bicycles to use the lane and move along the signals without stopping every 280 feet. The short cycle length is also important in the condition that you have a high percentage of turning traffic that can result in queue spillback between the intersections. Short cycle lengths give an opportunity to keep traffic moving. There's a longer debate on short cycle lengths, but the important element of block spacing is a big part of that debate.
NOTE: for some reason this figure is not available on the FHWA version of the Signal Timing Manual. I have confirmed that this link has a proper copy of the graphic.