First, A Teaser: You are driving on Pt. San Pedro Road approaching the signaled intersection at Loch Lomond Drive/Lochinvar Road (aka Loch Lomond Marina). Just as you are arriving and for absolutely no apparent reason at all – not a single vehicle or pedestrian on Lochinvar or Loch Lomond Drive – the light turns red and you have to stop. Why?
Techie Talk:This is a major re-write to a similar post I placed on Nextdoor some time ago. Forgive me for the length of this post, but it is difficult to fully explain what is happening in just a few words. To keep my typing finger from tiring, I will use some abbreviations: PSPR (“Pt. San Pedro Road”) and NS (“north-south” for the cross street to PSPR at this intersection – Loch Lomond Drive in The Strand and Lochinvar Road in Loch Lomond).
Imbedded in the roadway are traffic detectors which communicate with the signal controller, which is programmed to control the traffic signal timing. On PSPR, there are two detectors in each lane in each direction approaching the intersection; advanced detectors (ADs) that are about 200-230 feet from the intersection and limit line detectors (LLDs) just before the intersection. On the NS roadways, there are only LLDs, no ADs. Trust me, this is all important!
The operation of the signal for PSPR vehicles:
- The signal stays green for PSPR when there is no NS vehicle or pedestrian traffic.
- The minimum length of time the signal is green for PSPR is 15 seconds and the maximum is 40 seconds (but only if a vehicle has tripped one of the NS LLDs).
- A “gap time” of 3.5 seconds is set for the expected length of time it takes for a vehicle to travel on PSPR from an AD to the LLD. Each time an AD is tripped by a vehicle on PSPR while the signal is still green on PSPR, that green is extended by 3.5 seconds (i.e., the time goes from 15 seconds to 18.5 seconds to 22 seconds, etc.). This time extends up to a maximum of 40 seconds, but longer if no vehicles have tripped the NS LLDs.
The operation of the signal for NS vehicles:
- The signal stays red for NS unless a NS LLD is tripped.
- The minimum time the signal is green for NS is 9 seconds and the maximum is 20 seconds.
- If a vehicle trips a NS LLD, the signal changes to green for NS only afterthe timing has expired on PSPR (see previous section).
- The NS LLDs are about 20 feet long. As long as vehicles are detected within the LLD space, the signal will remain green for NS up to a maximum of 20 seconds before returning to green for PSPR. That would accommodate up to approximately 4 vehicles waiting in the NS direction.
What does all this mean? Let’s take a few scenarios:
- No NS Traffic:The signal would stay green for PSPR.
- Occasional NS Traffic:The signal would be primarily green for PSPR with the occasional change to green for NS for 9 seconds.
- Light but Consistent NS Traffic:Suppose one car arrives NS every 9 seconds but there is constant PSPR traffic. Then the signal would be green for PSPR for 40 seconds, then green for NS for 9 seconds (repeat – 40, 9, 40, 9, etc.).
- Worst Case Heavy Traffic:For heavy traffic in all directions, the signal could be green for PSPR for 40 seconds, then green for NS for 20 seconds (repeat – 40, 20, 40, 20, etc.).
Operation of the signal for pedestrians:
The operation is a bit different to accommodate the slower pace of a pedestrian versus a vehicle. When a pedestrian pushes the button, it is similar to when a vehicle trips the LLD except the length of the NS green is 23seconds instead of 9 seconds.
Now for the teaser. Why does the light change to red on PSPR when there is no NS traffic?
While this appears to violate all that I have said up until now, that is not the case. What can happen here is a sad result of the fact that the NS roadways do not have right-turn-only lanes. The result is that the LLD is tripped by any vehicle approaching the intersection on the NS roadways. If that vehicle is turning right and sees that there is no cross traffic, then that driver will turn right and drive on their way (right-on-red being legal in this state). Depending on the timing of the light at that moment, several seconds could pass (even up to 40 seconds!) before the light will change to red for PSPR because a NS LLD was tripped.
To an approaching vehicle on PSPR at that moment, it would appear that the light turned red for no reason whatsoever since the NS vehicle that tripped the signal will be long gone! Alas! Whether the NS roadways are wide enough to accommodate two lanes (one for right-turn-only) and the difficulty and cost of implementing changes to the in-road LLDs and signal lights is beyond my pay grade, as is the effect that would have on overall signal timing and delays. My guess is that this is way too costly and complicated to justify changes to this intersection.
People Driving Badly
A new issue has been generating discussions on Nextdoor due to a recent accident at the intersection and, possibly, other near misses. While there are left turn lanes on PSPR, in-road detectors and existing signals do not have a left-turn-only phase. While such a capability would probably reduce the likelihood of an accident involving someone turning left into oncoming traffic, it would also have the result of increasing even further the delays at that intersection for traffic going straight thru the intersection. I will leave it to others to discuss the relative merits of safety versus delay and related costs. However, shortly after this recent accident, the S.R. Public Works Department did install several “Yield to oncoming traffic on green” signs in the turn lanes on PSPR. Hopefully, that will cause better driving behavior and reduced accident risk.
I hope this was reasonably understandable, helpful and not too convoluted.
Alan Schaevitz
Board Member and Webmaster
Pt. San Pedro Road Coalition