Transit Signal Priority(TSP)

Transit Signal Priority (TSP)

• Problem: Transit vehicles are slow

• Problem: Transit vehicles are effected even more than cars by traffic lights– The number of times you stop at traffic signals

has a huge impact on travel time

Transit Signal Priority

• Solution: Give transit vehicles green lights on arterials

• Priority Versus Preemption (emergency vehicles)

Transit Vehicle Priority

• Concerns with TSP: – Too much priority causes excessive side street

delay– Giving transit vehicles priority messes up

progression– Messed up progression slows buses even more– Roadway agencies don’t want to lose control of

arterial

Solution

Find a graceful way of giving transit priority

1) Only change signal timing a little

2) Only change signal timing occasionally

Change Signal Timing A Little

1) Hold the green just a little longer to get the bus through the intersection

2) Shorten the side street green time to get the green for the bus to occur more quickly

3) Use a new traffic signal algorithm

Most Common Signal Logic Implementation

Long Green or Short Red

Only Change Signal Timing Occasionally

• Only late buses get priority

• Only special (Express?) buses get priority

• Only a few (random) buses get priority

• Priority is only available at some intersections

• Priority is only available on some corridors (bus priority corridors)

• Only full buses get priority

Limited Timing Changes

• Each of these approaches has a different impact on the combination of:– Size of improvements to transit– Size of impact to traffic– Distribution of those improvements / impacts– Amount of information needed to run the

system

TSP

Let’s examine how TSP works

TSP Basics

1) Identify bus as it approaches intersection2) Determine if bus deserves priority3) Determine if signal is willing to give

priority4) Determine if priority is needed (is signal

green already?)5) Change signal6) Let signal return to normal operation

Identify Approaching Bus

• Manual– Will driver’s use the system correctly?

• Automatic

Identify Approaching Bus

• Beacon (signpost)

• GPS + communications– To central?– To roadside?

• On-board dead reckoning– With DSRC

Determine if Priority Deserved

1) Which buses get priority?• If all buses = easy• If specific routes / runs = need data transfer

2) Transfer data to• The bus• The signal controller / interface• A central location

What Buses Get Priority?

• If only specific buses get priority, then there must be a connection between– Vehicle ID, – Daily Vehicle Assignment table (route/run), – Route/run and signal cabinet location– Maybe a “priority table,” and – Clock

What Buses Get Priority?

• Late Bus– Requires Bus ID + Route / Run +– Detailed knowledge of

• mid-point schedule times, and

• current location

What Buses Get Priority?

• Only Full Buses

– Requires a current passenger count

What Buses Get Priority?

• Limited number of buses (of any kind):

• Signal controller must keep track of– How many priority calls have been given, or

– When was the last priority call made?

Signal Controller Checks

• Is controller willing to give priority?– Limits set by roadway agency

• Number of priority calls per hour

• Time required between calls

• Is priority needed?– What phase is the controller currently in?

Signal Controller

• If priority is permitted & needed

• Select appropriate action:– Hold green time– Limit other green times

• No pedestrian calls

• Shorten side street green allowed

– Actuated signals

– Fixed time signals

– Other

Let Signal Return to Normal

• The more quickly this happens the better

• Exit detection versus timing

• Timing is dependent on speed of the vehicle and location of detection zone

Advanced Detection

Exit Detection

Presence Detection

Signal Controller and Interrogator

Mainline Signal

Bus Detector

Intersection

Detection Zone Location

• Where is the bus stop?

• Advanced warning?– Speed of processing (detection to priority call)

• Exit zone– Reduces time taken from side streets

Transit Signal Priority

Puget Sound Regional Implementation

Standard TSP Design

• Hardware– AVI Tag + Reader, or– Transmitter + receiver– Interface with traffic signal controller

• Interfaces also with central for route/run priority information

– Extra readers to• Give advance knowledge of bus approach

• Indicate bus has arrived at intersection

• Advise signal when bus has left intersection

System Configuration

TagAntenna

Interface Unit

Tag Interface Unit for Dynamic Data

Reader Cabinet

TSP System Components

System Data Flow Diagram

How and Why Do We Use TSP

• Keep buses on time– Happy riders– More riders?– Reduced transit cost (don’t need to build in

time to the schedule)– Improves transfer timing (faster schedules?)

How and Why Do We Use TSP

• Allows speeding up bus schedules

– Saves money (fewer drivers / buses?)

– Makes transit more attractive (more riders?)

Highcustomerbenefit

Lowcustomer

benefit

Low financialbenefit

High financialbenefit

Strategy A(fewer stops)

Strategy D(improved on-

timeperformance)

Strategy C(consistently

faster average busspeeds and/or

consistently lessvariable travel

times)

Strategy B (ocassionalfaster express service)

Operating Strategies and TSP Benefits

TSP Issues

• How much delay (mostly on side streets) does the priority system create?

• How many signal priority calls can be made?• How much is exit detection worth?• How do we control which buses get priority?• What benefits do we get from TSP?• How do we measure the benefits?

TSP Issues

• Side Street Delay– Effected by the type of priority calls

• Do we skip phases? Which phases?

• Do we shorten phases? Which phases? (peds?)

– Number of priority calls– Size of side street versus priority direction– Existing delay on side street

TSP Issues

• How many calls can be made?– Side street delay– Effect of calls on progression– How many calls actually change the signal

timing?– Can we monitor what actually happened?

TSP Issues

• Is exit detection worth the cost?– Importance of side street delay

• Cars

• Pedestrians

– Importance of cost– (Note: Light rail versus bus and the issue of

throughput)

Approximate TSP Costs per Approach

• Assumes underground installation• Plus system engineering, integration, & on-bus equipment

Without Exit Detection With Exit Detection

Design $2,400 $2,400Construction* $6,000 $7,000Reader Hardware $8,000 $16,000Controller/Interface $5,000 $5,000

$21,400 $30,400

TSP Issues

• What benefits do we get?– Increased transit speed– Increased transit reliability– Both speed and reliability

TSP Issues

• How do we measure the benefits / costs?– Intersection delay?

• Vehicle delay• Person delay (needs transit ridership)

– On time performance• AVL / TSP calls / other

– Scheduled route time• Bus schedules• Requires feedback to the scheduling department

TSP Issues - System Management

• Given all of the above choices, once the system is built:

How do we know it is working optimally?