DPW Pine Customer Service ha assegnato questo problema a Traffic Signal Engineer

This issue was recategorized from Traffic/Street Signs: Report Problem to Traffic Signal Problems.

This issue was recategorized from Traffic Signal Problems to Transportation and Parking Improvement Request.

Rule of thumb for left turn signals is that 2-3 cars are left stranded each cycle over a period of time. Not an occasional occurrence, but a consistent, ongoing circumstance that happens say for an hour. Morning is a tough time on the avenue. An arrow would help cars turning left, but it comes at the expense of delaying oncoming traffic. One or two cars at a time are probably not enough to tip the scale. It is usually assumed that one can turn left on yellow. We would have to watch and do some counts before it could be justified.

Our apologies for the delay in getting a response to this item. We wanted to make sure we got it right. After field observations, research and discussion, we do not believe that a left turn arrow would improve the overall performance of the signal control at Shore Road.

That said, because we are living in extraordinary times, it would be fair to wonder if volumes are unusually low due to people staying at home because of COVID-19 and the lack of school traffic. However, looking at some log files from late 2016 to 2018, the volumes were comparable to what was manually observed a couple of weeks ago.

The following details our observations, thought process and references to industry standards that support the conclusion that a left turn arrow isn’t needed.

Generally, a left turn arrow is justified if left turning (LT) vehicles cannot safely make the left turn against oncoming traffic over a period of time. “Over a period of time” meaning cycle after cycle the car cannot make the left turn. The occasional occurrence is not enough to support stopping other traffic at the intersection. The problem can be defined as whether adequate gaps exist in the conflicting traffic or not. Generally, if there are not enough gaps in the conflicting traffic and this leaves 2-3 cars stranded every cycle that situation would/could support an arrow. One or two vehicles are expected to be able to make it through on the yellow, possibly leaving 1 stranded.

To assess the situation at this intersection, two traffic flow observations were made. One on the afternoon of Wednesday, July 22nd, the other the next morning, the 23rd. The attachment summarizes; the times noted are the start of each 15 minute count period. Counts were taken of northbound left turning (NBLT) vehicles and southbound (SB) vehicles. Other movements do not affect this determination, so nothing else was counted.

Along with the flow counts, the number of NBLT cars stranded each cycle was observed.

Wednesday was a very pleasant, sunny afternoon. The heaviest volume observed was from 16:30-16:45, with a combined total of 142 vehicles. Spread out evenly over 15 minutes, this would be a car arriving every 6.3 seconds. Of course this is not what happens. Cars tended to arrive in groups of 2-6 (SB).

The busiest pace occurred at about 16:50, when it became harder to keep up with the counts. SB traffic was steady, and NBLTs were having to wait for gaps. However we observed only one NBLT car stranded on red after waiting on green. This was the sole occurrence that afternoon.

The next morning it was raining steadily. The thought was that maybe this would create a skewed observation. On the other hand, it was not going to prevent normal commuting, and perhaps more people would be using cars instead of bicycling. Because bicycles were counted as SB vehicles and since NBLTs have to yield to bicycles too, our belief is that that the weather had no practical effect. During this period, no cars were stranded.

Because only one left turning car was left stranded in the afternoon and none the following morning, under what appeared to be “typical” traffic flow conditions, we don’t believe that a left arrow would improve the overall performance of this intersection.

As additional reference, the Virginia DOT has a publication that provides guidance for left turn phasing:

https://www.virginiadot.org/VDOT/Business/asset_upload_file523_149245.pdf

On page 18, it describes volumes at which a left turn phase should be considered. A quick analysis uses a "cross product" determination. Hourly oncoming traffic volume, divided by the number of lanes, is multiplied by left turn volume. With a cross-product of 50,000 or more, gaps are fewer, and a left turn signal should be considered. At Shore Road, the highest volumes observed yielded a cross-product of 11,700, well below the threshold suggested in this guidance document.

The Department of Public Works has resolved this request. Thanks for helping to keep Burlington a great place to live and work.

Thank you for looking into this so closely.