Synchro/HCM results - unsignalized adjacent to signalized 1

[marks55]

We are modeling a 2-lane arterial with peak 1-way hourly vols in excess of 1200 vph (2200 2-way vol) in Synchro v7 (HCM method) to determine the effects of adding a signal at one or more intersections.

Results so far show that when one signal is added, all of the ajacent (<300 m away) intersections "blow up", i.e. sideline delay (TWSC) increases drastically. HCM methodology suggest ther may actually be a benefit to signals downstream of signals, but our results show differently.

Can anyone explain this phenomenon?

Thanks.

 

[ACtrafficengr]

Other than a problem with the analytical model, the only way I can think that this would happen to a "downstream" intersection would be if it had heavy left turns, and the signal queued through the intersection.

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

 

[teeman]

Without knowing the details of the network only some general observations are possible.

Under the best of conditions traffic signals can only process approximately 1000vph per lane. At some locations due to approach grades or %trucks, operations can start to break down at around 800 vph per lane. A 200 car queue is approximately a mile long.

Platoons created by the introduction of signals can have a significant adverse impact on adjacent unsignalized sidestreets. During periods of significant mainline flows the passing of platooned traffic from one direction and then the next may eliminate gaps for adjacent sidestreets. Also, platooning dissipates over time and distance. If the sidestreet is far enough from the signal it may experience a continuous flow of mainstreet traffic with no servicable gaps.

This is further complicated by the nature of the HCM unsignalized methodology that generally overestimates the required gap and the associated delay.