The Valley Metro light-rail line carries passengers 20 miles from Mesa, Ariz., to its western terminus in Phoenix, a trek spanning three cities. It rolls past Arizona State University, major sports stadiums and bustling business corridors, catching green lights most of the way.

Like Metro Transit’s Green Line in Minnesota, Valley Metro Rail runs down the center of a busy commercial avenue. Unlike the frequently delayed Green Line, it completes the 28-station trip in a reliable 65 minutes, or little more than three minutes per mile, thanks in large part to the priority it enjoys at traffic signals.

“We don’t hear a lot about delays,” said David Schwartz, executive director of the Friends of Transit advocacy group in the greater Phoenix area. “I think it’s been well received.”

In contrast, the 11-mile, 23-station Green Line between downtown St. Paul and downtown Minneapolis has been averaging about 52 minutes, or nearly five minutes per mile.

“I’ve ridden it, and it stops at every light and every station. It doesn’t seem reliable in terms of timing,” said Jessica Treat, executive director of the transportation advocacy group St. Paul Smart Trips.

Metro Transit officials and transit advocates say the main problem is that St. Paul’s traffic signals are not syncing well with the Green Line, a delay that many officials are laying squarely at the foot of the city.

“You can see where trying to get those signals synchronized could easily save five minutes off the schedule,” said Metro Transit General Manager Brian Lamb. “One trip we might be running 48 minutes, and the next trip … you might be significantly higher than that.”

Average weekday ridership on the Green Line through Aug. 8 is 30,970 passengers — 12.5 percent ahead of 2015 projections. With students returning to the University of Minnesota and other metro colleges next month, ridership is expected to get a further boost.

But the $957 million line has come under scrutiny for its lack of predictability and punctuality. End-to-end travel times — once projected to take 40 minutes — are clocking in well beyond Metro Transit’s official time schedules of 48 minutes.

In practice, trains are averaging 53 minutes westbound and 51 to 52 minutes eastbound. Some individual trips are running closer to an hour.

The Phoenix line is on time 95 percent of the time.

Minnesota’s Green Line: 55 percent, including trains up to four minutes late.

Treat said St. Paul Smart Trips wants to present public transit as an efficient and attractive alternative to commuting in cars but trip time has made light rail tough to promote. A late train, for example, may result in a missed connecting bus, transit advocates note. The system operates best when it is reliable.



STOPPING AT LIGHTS

Lamb said the Green Line’s biggest time problem is city-owned traffic lights.

Lights in Phoenix anticipate and prioritize light-rail cars. The Green Line is outfitted with similar technology, with far less effective results.

Throughout the system, the goal is for detectors under the track to help traffic signals better time the length of their red, yellow, green and left-turn sequences. Those sequences cycle within a two-minute window, but the length of any individual light can vary depending on factors such as traffic on cross-streets and pedestrians pushing walk buttons at crosswalks.

Lamb said a recent review found that detectors picked up 90 percent of the light-rail movements, a number he expects will improve. But the biggest challenge, he said, remains the traffic signals themselves.

The Green Line’s frequent, unscheduled stops at red lights have thrown it out of sync with connecting bus routes and official travel schedules, and the line occasionally interferes with northbound Blue Line departures near the Minnesota Vikings stadium.

At its fastest, the Green Line is running about 35 mph. Lamb believes it could get up to 40 mph if it didn’t have to decelerate as often.

Metro Transit shared traffic data last week with city engineers in hopes of testing a new strategy at three low-volume University Avenue intersections — Victoria, Grotto and Chatsworth streets. The light rail would gain more authority over signals in that stretch, though not as much as emergency vehicles.

“It takes about a week to run it through their models, after we provide the base data,” Lamb said. “We’re hopeful.”

BALANCING ACT

David Levinson, a professor of transportation engineering in the University of Minnesota’s Department of Civil, Environmental and Geo-engineering, says St. Paul had plenty of time to perfect Green Line traffic signals during six months of test trips.

He suspects the decision not to give the Green Line nearly as much priority at traffic signals as the Blue Line is mostly political. When the Blue Line debuted in 2004, cars lined up for long waits on Minneapolis cross streets. City engineers in St. Paul feared a repeat.

“I think the city could do more,” Levinson said. “I think the city knew about this for a very long time. I think the city was scared of the very long signal times on Hiawatha Avenue. … They were reluctant to give as much priority.”

Kari Spreeman, a spokeswoman with St. Paul Public Works, said the city is committed to making sure bicyclists and pedestrians can cross the avenue, cars can make left turns, and the light rail can go by. It’s a lot to balance.

“We have a team of traffic engineers working on the system every day and are continuing to work closely with Metro Transit to tweak the system,” Spreeman said. “Our goal is the same as it has been from the beginning — to strike a balance.”

Greg Hull, an assistant vice president with the American Public Transportation Association, said he’s seen other cities wade through similar questions about how to balance major transit investments with competing traffic demands.

“The challenges you’re facing in Minneapolis-St. Paul are not unusual for what you’ll find in most cities,” Hull said. “They become political decisions, and it becomes a matter of local jurisdictions needing to determine what’s in their best interest.”

Some transit engineers say the conflicts between cross-traffic and public transit aren’t always as significant as they are perceived to be.

A 2003 study based in Fairfax County, northern Virginia, found that giving buses priority at intersections through extended green lights improved their reliability without significant impacts on traffic at cross-streets. In fact, the traffic queue on the side streets increased by one vehicle.

“It’s important to recognize there’s a trade-off,” Levinson said. “That said, there’s going to be a lot more people in a train than in a car at any time, so the trade-off should favor the train.”

Nate Khaliq, a former firefighter and neighborhood activist who lives in the Summit-University neighborhood, said he was surprised that the train doesn’t already get priority at traffic lights.

“I would have thought they’d have all this stuff together, when you put $1 billion into a public transportation project,” Khaliq said. “It certainly wouldn’t bother me to wait a little longer at stop lights.”

PREDICTIVE PRIORITY

In the Phoenix area, the Valley Metro system has been able to sync urban light-rail trains with green lights “most of the time,” agency spokeswoman Susan Tierney wrote in an email.

Valley Metro’s “predictive priority” technology works with traffic signals in Phoenix, Tempe and Mesa to continuously update and adjust traffic lights as light-rail trains approach. But trains do still idle in front of red lights from time to time.

“On a 20-mile, in-street operation, we do not take full priority over the signals because of the severe impact it would have on roadway traffic,” Tierney said. “With trains coming every 12 minutes, through 148 intersections, making 225 trips per weekday, full signal pre-emption would create daily and valley-wide gridlock for motorists.”

Where Seattle’s light-rail line runs along streets, the train hits green lights “nine times out of 10,” said spokesman Bruce Gray.

Their system tells upcoming traffic signals how fast the train is moving, allowing the signals to time their light sequence accordingly.

“We had a fair share of tweaks we needed to do to the signal system when we first opened,” Gray said. “It probably took us a couple of months. … It doesn’t hold up the side streets too much.”

Denver’s light-rail system also “receives a green the majority of the time,” said Scott Reed, a transportation district spokesman, in an email.

The district, city and a consultant worked closely together before the September 2011 debut of the current four-car operation, which required restructuring the traffic light timing.

“Train movement was the first priority,” Reed said.

Calling the timing effort a “unique process,” he cautioned that “one size does not fit all.”

Some of the most popular systems in the country receive no priority at traffic signals. With the exception of three sharp turns along Canal Street, streetcars in New Orleans get no signal pre-emption.

“We don’t do anything special for them,” said Kenneth Songy, a maintenance engineer with Veolia, which operates one of the oldest streetcar systems in the country on behalf of the New Orleans Regional Transit Authority.

After nearly 120 years in operation, the green streetcars that rumble down some of Boston’s busiest avenues may soon get priority at traffic signals for the first time, thanks to a GPS-based technology being installed this month.

“It’s taken a while, because MBTA is not a city agency,” said Massachusetts Bay Transportation Authority spokesman Joe Pesaturo. “We’ve had to work with the city of Boston, because they control the traffic signals. But we’re finally there.”

Elizabeth Hernandez contributed to this report.

Frederick Melo can be reached at 651-228-2172. Follow him at twitter.com/FrederickMelo.