Madison Transit Signal Priority, and what I've learned

We've had our BRT line open here in Madison for over a year now. I wanted to share some lessons learned working as a traffic engineer on the project as it relates to "TSP".

Before I begin, I want to clarify that from now on when I use TSP, I'm using it as a broad term of ways traffic signals can prioritize transit (in our case buses). I think a lot of times people think TSP is only extending the green, truncating the red, and involves some device on the bus and some device in the cabinet. For that, I will be using the term Signal Control Priority (SCP), as that's what our controllers refer to it as.

What I've learned after working on our system, talking to other engineers/transit planners, and various other people involved in operations and advocacy is that TSP comes in many different styles, and transit can be best served when all agencies involved are onboard and understand what strategy best fits where.

So here are the strategies we've utilized and what they do. 1. Centralized SCP: This is the common one people think about. Some form of communication between the bus (gtfs data packets) and the central signal system. The central signal system picks up this data and any requests, then pushed down the requests to the traffic signals. It's nice, it's good for mixed traffic conditions, and can use any of the NTCIP priority levels if desired.

It is limited though in it's ability to make more specific signal adjustments but can also have rules such as only providing the priority request if on or behind schedule.

2. SCP Detectors: Some input into the controller from the field that with some logic steps/programming can provide a little more nuanced signal operation. It could simply be a loop detector in a bus lane before and/or after the bus stop that turns on an SCP detector, and you can do what you want with that.

It's limitation is that you have to have good reliability that when that detector goes off, it's a bus. So median bus lanes are good...shoulder bus lanes could be problematic. However, with new video/radar/lidar systems, there are ways to determine if a vehicle in mixed traffic is a car, bus/truck, or whatever by size (articulated bus vs standard bus even). So that has the potential to solve these methods biggest issue.

3. Peer to peer communication: Not really transit specific, but with this you can have signals look at other signals detectors and signal status. IF the bus detector is on and the phase the bus proceeds on is green, you can place an advanced SCP call at the downstream signal. You could even call the side street ped phase this way. Really, you can do quite a bit with this system. But it requires communications to all signals.

4. Phase insertion: The way we've handled this is to make say a left turn phase for a bus an overlap, then place dummy phases throughout the signal cycle. When the detector is on, it calls all of those phases, then when the bus leaves, all calls are dropped. That way, rather than waiting for that one phase per cycle, the bus might have 2, 3 or more phases per cycle and is never more than a phase away from it's phase.

This can also be paired with SCP detectors, so when the bus pulls up, the controller basically speeds runs through the current phase and goes into the next available bus phase. This is actually my favorite and is really fun to watch. It works great at protected only transit movements.

5. Phase omission/rotation: Similar to rotation, but if you have certain situations where you want to skip certain other phases, this can be used. So a near side stop arriving on a red, you could skip the opposing left turn phase to get back to the bus phase sooner but then provide a lagging left turn phase so those drivers still get their turn phase.

6. Signal coordination: In many cases where signals are close together, traditional SCP doesn't really help all that much unless all signals can guarantee passage, which is unlikely. So instead, where our buses run in tight signal dense areas such as around the square, the signals are just coordinated for bus movements between stops. The bus leaves a stop, might stop at the next signal, then gets a green the following 2-3 signals until it's next stop...then starts over.

8. Coordinated vs free: SCP systems of any type seem to run much better if the signals are running free vs coordinated. So look for opportunities to take signals out of coordination. Even if it's just a few, it could help reduce stops at that signal.

9. Misc. stuff: not related to traffic signals, but things like station placement, lane assignments at intersections, allowing buses to proceed thru on right turn only lanes...these sorts of things also have a lot of impact when scattered across a system in smart ways.

I've really enjoyed working on our system, meeting new people as part of the process, and trying to learn new ways to help our system as well as spread the word on how to implement these strategies in other systems. I want to call out a few agencies for having great staff and they should be praised for their efforts (PBOT, MBTA, IndyGo, and Minneapolis),

I'm excited to see what we can do with some upcoming detection systems and to continue to see what a lot of other agencies are doing that don't really get the public attention they deserve.