Priority signaling

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Priority signaling

Postby typesix » Fri Oct 27, 2017 11:32 am

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Re: Priority signaling

Postby MBTA3247 » Fri Oct 27, 2017 4:42 pm

About time. Didn't Brookline wire up the signals on Beacon Street for signal priority a decade ago?
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Re: Priority signaling

Postby F-line to Dudley via Park » Fri Oct 27, 2017 4:59 pm

Not "wire up" per se, but all the new traffic signals on Beacon are computer-controlled such that the whole corridor can be programmed to coordinate signals and prioritize queue-flushing. And the signal boxes can accept input from add-on trolley or bus sensors. Huntington, during its major road reconstruction, also got digital signaling that has the ability to be reprogrammed for preemption. The only place that doesn't exist is on most of Comm Ave. west of Carlton St., which still has 'dumb' analog signals. If you've ever been waiting for a crosswalk signal and can hear the "ka-chunk!" sound of the mechanical traffic light switches turning in the nearest signal box...that's an old-school analog setup. Those are due to be replaced from BU Bridge to Packards Corner with the Comm Ave. Phase II reconstruction project kicking off imminently, enabling more of the corridor to get prioritization (I think Brighton Ave. also got digital signals during its 1999 corridor reconstruction). Packards to Warren is dependent on the stalled Phase III reconstruction, which isn't happening any time soon due to budgeting. And I guess further up the hill to BC the City of Boston will play it by ear re: what signals they choose to replace, although transit priority isn't nearly as high-stakes out there on the lighter-traffic portion of the line as it is through BU.
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Re: Priority signaling

Postby The EGE » Fri Oct 27, 2017 10:51 pm

Holy crap, Boston still has the old-school rotary signal timers? They're museum pieces here in SF.
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Re: Priority signaling

Postby F-line to Dudley via Park » Fri Oct 27, 2017 11:23 pm

Hundreds of them city-wide. It would take a lifetime to wholesale-replace them all, so BTD doesn't have much choice but to keep maintaining the mechanical parts as year-to-year programmed replacements are a drop-in-the-bucket vs. the number of legacy installs still out there. BU Bridge to Packards has several rotary-timed traffic signals, and Packards-Warren is mostly solid-state analog (i.e. also non-programmable) from an early-70's cycled replacement on that part of the corridor.

The rotary ones are identifiable by the old-school rounded iron control boxes (yes, that one on the corner of Babcock does make a resounding "ka-thunk!" sound when it's cycling the traffic and trolley signals). The solid-state analog boxes from the 1970's-80's tend to be cheap aluminum with air circulation vents unlike the rotary boxes that trap all their heat. The newer digital boxes tend to be either heavy-duty stainless steel or retrofitted old aluminum boxes modded with additional vent slits.
Last edited by CRail on Sun Oct 29, 2017 2:18 am, edited 1 time in total.
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Re: Priority signaling

Postby CRail » Sun Oct 29, 2017 2:24 am

MBTA3247 wrote:Didn't Brookline wire up the signals on Beacon Street for signal priority a decade ago?


Yes! And when they did it they offered priority to the T. The T's response was that they wanted to study the affects it would have on the service before spending money on the extra hardware, and they wanted Brookline to fund the study. Brookline (rightfully, in my opinion) told the T to pound sand, and so nothing happened. Fast forward the decade, and eureka! We could have signal priority!

No kidding... why didn't anyone think of that before!? 🤦🏻‍♂️
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Re: Priority signaling

Postby Disney Guy » Sun Oct 29, 2017 4:55 pm

Transit signal priority might not work that well with bus (or trolley) stops before the intersection (near side stops). If an operator opens the door again to board another passenger, then the time reserved for priority passage of the bus or trolley gets wasted and other traffic is delayed with no benefit to transit.

What is needed is an on board system whereby the transit vehicle, notably a two or three car train, cannot signal its presence to the traffic signal if any door is open and the motorman must hold down a button to request priority and that also prevents any doors from being opened.
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Re: Priority signaling

Postby diburning » Sun Oct 29, 2017 5:17 pm

There perhaps may be an even simpler solution than that. At the stops where the stop is at an intersection, they could simply move the platform back (along the ROW) a few feet, and add the trigger/sensor a few feet past the platform. That way, the operator would not open the door since the car is no longer completely on the platform. This also solves the problem of triggering the sensor at a station stop. It probably costs a bit less to repave a few platforms than it would to retrofit the entire fleet with buttons
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Re: Priority signaling

Postby MBTA3247 » Sun Oct 29, 2017 9:25 pm

Could that sort of repaving be done without triggering a full ADA-compliant rebuild?
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Re: Priority signaling

Postby F-line to Dudley via Park » Mon Oct 30, 2017 12:52 am

Disney Guy wrote:Transit signal priority might not work that well with bus (or trolley) stops before the intersection (near side stops). If an operator opens the door again to board another passenger, then the time reserved for priority passage of the bus or trolley gets wasted and other traffic is delayed with no benefit to transit.

What is needed is an on board system whereby the transit vehicle, notably a two or three car train, cannot signal its presence to the traffic signal if any door is open and the motorman must hold down a button to request priority and that also prevents any doors from being opened.


Certain optical sensors work that way. An infrared camera mounted on the top of the trolley signal detects the shape of the vehicle as it approaches and can trigger a signal cycle based on proximity. When there's a station stop the detector notices the change in front marker light aspect on the trolley when the doors close and the flashing lights go solid, and uses that as the trigger to scramble a go signal. Fairly simple and elegant in execution as it doesn't require any change in ops practices or installation of any 2-way radio infrastructure. Brookline assumed the T was most likely to adopt a system like this when it was doing over all the signals on the Beacon corridor.

These optical sensors can also work in mixed traffic with trolleys or buses, likewise picking up the transit vehicle by shape and marker aspect (if it's a bus, then the marker aspect can be replicated by always putting hazard flashers on during a stop). Not as quick on-the-draw as a detector aimed squarely at a dedicated ROW because there's usually a programmed delay to root out false positives, but it works the same way and is fairly robust in its simplicity.
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Re: Priority signaling

Postby Disney Guy » Mon Oct 30, 2017 8:03 am

The system described "can make the green arrive up to 14 seconds earlier and can extend the green up to 8 more seconds." I agree that the benefits would not be that great of the overall red light could last say 60 to 100 seconds.,

What is needed is to be able to insert short green lights for transit in between the subphases of a long red light. For example if, on an east-west trolley route, a cross street has separate phases for northbound and southbound mixed traffic, an east-west trolley only phase could be created on demand between the northbound and southbound cross street phases. In addition a trolley phase could occur both before and after a left turn phase for the same street as the trolley route instead of the either-or (always before vs. always after) we use today. Now, these insertions are not going to occur in every possible place on every cycle so the impact on general traffic should not be that great.

This could eliminate, say, a 25 to 40 second chunk of waiting time from the trolley route.

A hypothetical (long and technical) system is described here: http://www.cockam.com/serveme.htm
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Re: Priority signaling

Postby blackcap » Mon Oct 30, 2017 9:52 pm

The UTA TRAX system uses detector loops either embedded in the pavement between the rails or attached to on top of railroad ties for signal pre-emption in the areas where trains run in dedicated ROW’s in the medians of streets.
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Re: Priority signaling

Postby Komarovsky » Tue Oct 31, 2017 8:03 am

Disney Guy wrote:The system described "can make the green arrive up to 14 seconds earlier and can extend the green up to 8 more seconds." I agree that the benefits would not be that great of the overall red light could last say 60 to 100 seconds.,

What is needed is to be able to insert short green lights for transit in between the subphases of a long red light. For example if, on an east-west trolley route, a cross street has separate phases for northbound and southbound mixed traffic, an east-west trolley only phase could be created on demand between the northbound and southbound cross street phases. In addition a trolley phase could occur both before and after a left turn phase for the same street as the trolley route instead of the either-or (always before vs. always after) we use today. Now, these insertions are not going to occur in every possible place on every cycle so the impact on general traffic should not be that great.

This could eliminate, say, a 25 to 40 second chunk of waiting time from the trolley route.

A hypothetical (long and technical) system is described here: http://www.cockam.com/serveme.htm


Also an important consideration, being able to control the left turn signals at some intersections. There are several intersections where non turning traffic could be allowed to proceed, while the left turn lane could be held until the train has cleared the crossing.
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