Timing downtown one-way grids

[tradephoric]

Many downtowns in America have uniform one-way grids.  One example is Portland, Oregon.  The traffic signals in downtown Portland are timed so that traffic progresses in a slow but steady speed in all directions.  This is possible because of something known as quarter cycle offsets and is discussed by a Portland engineer at rEvolving Transportation:

Signal Timing Using Quarter Cycle Offsets in Downtown Portland
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.
http://koonceportland.blogspot.com/2010/10/portland-tribune-article-on.html

Visually here is an aerial model of Portland's downtown grid.  The turning movements have been removed to highlight the through platoon coordination.  Also there is about a 30 minute video of a guy cruising around downtown Portland to give a sense of how the signals are timed.
https://www.youtube.com/watch?v=7jGWdCknurM&t=60s
https://www.youtube.com/watch?v=-Z9IX-mPKeQ

I put together a chart that looks at the relationship between grid spacing, cycle length, and speeds that allow for good progression in all directions along a one-way grid.  In the Portland example, the blocks are spaced 280 feet apart, the signals are timed for 13 mph, and the signals have 60 second cycle lengths.  You can now use this chart to estimate how downtowns with different block spacings may time their signals.

   

*Only pedestrian friendly cycle lengths are included in this chart (ranging from 60 seconds to 100 seconds).  Shorter cycle lengths are considered more pedestrian friendly as it allows pedestrians to cross the street without having to wait very long.  But there is a limit to how short a cycle length can be.  Pedestrians still need enough to time to safely cross the street and cycle lengths of 60 seconds are about as short as you can practically run.  As the width of downtown streets gets wider, the cycle length must increase to allow pedestrians to safely cross.  Portland is capable of running short cycles the pedestrian crossing widths are relatively short, but this isn't possible in cities with wider streets.  Part of the reason why Portland has short crossing widths is because they have very short block spacing.   You don't need 6-lane wide streets to move traffic if there is a street every 280 feet apart. 

*Only grid spacing of up to 600 feet are included in the chart.  The practicality of downtown one-way street networks diminishes as the block size increases.  Salt Lake City has big block sizes of about 800 feet and the majority of streets in Salt Lake City runs two-way.

[tradephoric]

From Seth Kadish's VizualStatistix: "A block is by no means a standard unit of measurement. Depending on the urban plan, blocks can be square or oblong, and can vary significantly in side length. For this plot, I measured the median length of downtown blocks in six cities that have regular grid layouts – that is, the dimensions of their city blocks are consistent. I've included blocks per mile along the outer axes, and the ratio of short to long dimension for each city. For this value, one represents a perfect square while smaller values indicate more oblong blocks."