ACHD's Road Wizard Sunday, December 22, 2019

When David Letterman had his late-night talk show, his "Top Ten" list was a nightly occurrence. If ACHD had its own "Top Ten" list of frequently asked questions, this would surely be included.

Unfortunately, just because a request may meet the "frequency criterion" doesn't mean it's a good idea. Though it seems simple, a change like this has significant consequences for the entire area, as the Downtown Boise signalized network is designed to work in synchronization. During a typical weekday afternoon and evening commute timing plan, the cycle length (or the time it takes to provide a green indication to all intersection movements) is 70 seconds at the east end of Front Street from Avenue A to 5th Street, and 140 seconds from 6th Street to 13th Street. The longer cycle length accommodates the doubling of traffic that occurs on Front Street between 6th Street and 9th Street during the afternoon and evening peak travel period. This provides the additional "green time" necessary to move the larger number of westbound vehicles relative to north and southbound traffic.

Because the signal at 13th and Front is running a common cycle, if a "protected" left turn (green arrow) movement is added for the northbound left turn lane, time must be taken from either the southbound through movement or from Front Street. This ensures all movements are served within the 140 second cycle. The green time given to 13th Street has already been nearly "minimized" to what is required for a pedestrian to cross Front Street. Therefore, time provided for a protected left turn would, by default, need to come from Front Street. Unfortunately, based upon analysis and experience, this isn't a realistic solution, as taking even a few seconds away from Front Street can cause traffic to back up as far east as Avenue A and, in a "worst case" scenario, Broadway Avenue.

As Downtown Boise "fills in" with hotels, apartments, condos, and offices, maintaining the status quo is likely to become more challenging, not less. There are ways to improve traffic flow, but they tend to be fairly severe, such as prohibiting turn movements, and may have other implications to locations elsewhere within the network. Even though such measures are possible, it doesn't necessarily mean ACHD is considering anything like that in the near term.