Signal Priority for Transit is the Best Investment

Imagine going to a restaurant with a large group of 40 people, and you are all seated at the same table.  The restaurant serves their food family style, but you’re surprised to see that your server provides your table only a serving suitable for one person, like the adjacent tables of individual patrons dining alone receive. You ask the server where the rest of your food is and they respond that restaurant policy is to give each table the same amount of food, no matter how many guests.

Allegheny county’s traffic light implementation is just as absurd as this imaginary restaurant’s food policy.  The lights allocate the same amount of time to a single occupant vehicle as a public transit bus with 40 people.

Fortunately there is technology to fix this problem: giving transit vehicles priority at traffic signals.  Implementing signal priority is the single best investment the Port Authority can make.

Quantifying the Time Savings

Traffic signals are a significant source of delay for transit buses. A study of implementing signal priority in Davis, California showed signals delaying each bus between 38.7 seconds and 138.3 seconds, with an approximately 90 second delay being typical.  While a study is necessary to quantify the signal delay that is typical in Allegheny county, the Davis California study shows that there is potential for significant time savings.

A 2005 study funded by the department of transportation reviews several case studies of implemented transit signal priority. The results are summarized in Table 1.

Table 1: Summary of Transit Signal Priority Case Studies

System	Total Cost	# Signals	Cost Per Signal	Total Time Savings	Time Savings per signal
AC Transit Oakland CA	$325,000	62	$5,242	Estimated 9%
King County Metro	N/A	28	$35,000	5.5-8% reduction in total travel time	25-34% decrease in intersection delay
Los Angeles MTA	N/A	654	$30,000	19-25%(Note 1)
Chicago IL	$732,000	15	N/A(Note 2)	15% reduction in travel time
Pierce Transit Tacoma County Washington	$2,786,000	110	$25,763		Signal Delay reduced by 40%

A 2002 University of South Florida study documents the speed improvements for several transit signal priority projects.  It documents that a transit signal priority system in Phoenix, Arizona saved buses 15 seconds per intersection.  I had trouble finding other studies that expressed time savings by this metric as most only indicated percentage reduction in either signal delay or total travel time.

An Excellent Return on Investment

Signal priority is such a good investment that it pays for itself it saved operating costs, while providing even larger benefits to riders and the community.

In February the Port Authority was currently able to borrow at an interest rate of only 2.3%.  I don’t have information on how this interest rate may have changed since then, but I will use it in my analysis. Assuming a cost per traffic signal of $\$$35,000, and a 15 year amortization at a 2.3% interest rate,  upgrading the traffic signals would cost $\$$230.10 per month or $7.57 per day.  On page 12 of the FY2020 budget, it identifies the operating cost of a bus is $\$$188.43 per revenue hour.  So a traffic light with signal priority pays for itself if it saves the buses that pass through it each day a total of 2 minutes and 25 seconds.  

$\$7.57 \times 60 \frac{\textrm{min}}{\textrm{hr}}\times\frac{\textrm{1 hour}}{\$188.43} = 2.41\ \textrm{minutes}$

$0.41\ \textrm{minutes} \times 60 \frac{\textrm{seconds}}{\textrm{minute}} = 25 \ \textrm{seconds}$

If the time savings of 15 seconds per intersection realized in Phoenix could be replicated in Pittsburgh, it would require only 10 buses per day (5 round trips) for a route to pay for itself.  That’s before considering any fare revenue from additional riders.

There are two ways to look at this.  One is that signal upgrades not only speed up buses, they are also the most cost effective way to add service miles.  It is cheaper to pay for an upgraded signal, and use the saved time to increase the frequency or number of routes than it is to pay for more drivers and buses.

Another way to look at it is upgrading the traffic signals can pay for itself.  The Port Authority could reduce service hours, use the savings to pay for the signal upgrades, maintain the same service frequency with faster buses, and still have savings to spare for other improvements.

While I would personally prefer obtaining additional funding and expanding service, either option is better than retaining the existing unintelligent traffic signals.

The operating cost savings of signal priority makes this upgrade easy to fund, but the benefits are much larger.

The value of time saved to riders exceeds the benefit of reduced operating costs.  The time saved for a rider can exceed the time saved for a bus because signal priority can be used to improve schedule performance and headway management, reducing the time passengers have to wait at bus stops. The signal priority project in Los Angeles identified in Table 1 indicates that a majority of the time savings for passengers may come from reducing bus stop wait times.

The value of the in-vehicle time savings can be estimated as follows. According to the 2018 annual service report, there are an average of 34.2 riders served per service hour. To estimate the number of people on the bus at a given traffic signal, one also needs the average in-vehicle trip duration per rider.  I couldn’t find hard data on this, so for the purpose of my back-of-the-envelope analysis I will guess the average rider is on the bus for 20 minutes. 

$34.2 \frac{\textrm{riders}}{\textrm{hr}}\times \frac{1\ \textrm{hr}}{60\ \textrm{minutes}} \times 20\ \textrm{minutes} = 11.4\ \textrm{rider average bus occupancy}$

Assuming a time value of money of $\$$18.58 per hour [corresponding to the 2018 median hourly earnings], the value of riders’ time exceeds the benefit of reduced operating cost. 

$11.4\ \textrm{riders} \times \$18.58 \frac{1}{\textrm{hour}\cdot\textrm{rider}} = \$211.81\ \textrm{time value generated per hour}$

Additionally, speeding up the buses will increase the mode share of transit ridership.  This yields huge benefits, including reduced pollution, reduced congestion, better health and safety, and fostering development that improves land use.

Drivers may be concerned that transit signal priority will slow them down, but this concern is misplaced. The studies I’ve found (and linked in this post) of the effect of transit signal priority on cars have only looked at the localized effects on a converted intersection.  These have shown the delay to cars is small to negligible.  If signal priority is implemented broadly enough to drive a mode shift to transit, the reduction in congestion may result in a net time savings to cars as well.

Implementation

There are various implementations of signal priority.  Methods should be used that minimize both pedestrian and transit time delay, and concurrently implement intersection improvements for pedestrian safety.  In some intersections, queue jump lanes for transit may be appropriate.

The Port Authority currently has limited plans to implement signal priority.  Signal priority will be included in the bus rapid transit being developed between Downtown and Oakland.  This corridor, including segments extending to Squirrel Hill and Shadyside, has the highest bus ridership in the county.  Signal priority on these routes will provide the greatest benefit.

Pittsburghers for Public Transit has evaluated bus service improvements to the Mon Valley and Monroeville using a grant from The Heinz Endowments.  While their recommendations are still being finalized, the initial recommendations include signal priority.

These implementations will provide a huge benefit, but they don’t go far enough.  The benefits of signal priority are so large and clear that they should be implemented on every traffic signal that transit vehicles pass through.

Most traffic signals are owned by the municipality that they are located in.  Municipalities should proactively reach out to the Port Authority to express interest in collaborating on implementing signal priority.

With the COVID-19 crisis straining the Port Authority’s budget, it is important to focus on cost effective transit improvements. Signal priority is an investment that should be made.