It’s every driver’s dream: You’re on a busy city street and as you approach each intersection, the traffic signal changes to green.
Instead of stop and go, it’s go go go.
Minneapolis is in the midst of an upgrade for its traffic-control system that includes a new computerized system that can re-time traffic signals to make traffic flow more efficiently.
New signal timing is in place downtown; changes in the rest of the city will be complete by next summer. The $11.2 million project is funded by a federal grant and state, county and city money.
City officials say traffic flow downtown could improve by 10 percent. The system also allows the city’s Traffic Management Center to analyze traffic patterns and activate and turn off left-turn arrows.
The idea behind the synchronization of traffic signals is simple: Traffic congestion costs time and money and increases emissions that contribute to air pollution. So nudging vehicles to move along with fewer stops and starts is a positive step.
But some traffic experts have a counterintuitive view. Improving traffic flow, they say, encourages people to drive more.
“Any time you make it easier or faster or less congested on a road, the more people will drive,” says Dom Nozzi, a transportation consultant based in Boulder, Colo., who blogs at nozziwalkablestreets.com.
“When we introduce changes that make it less inconvenient or less congested, we induce new car trips that would otherwise not have happened,” he says.
That’s not the prevailing attitude among traffic engineers in most U.S. cities.
Earlier this year, Los Angeles completed a $400 million project that included synchronization of all of its 4,500 traffic signals. The city says a five-mile trip that took an average of 20 minutes without signal synchronization now takes 17.2 minutes. The average speed on city streets has climbed from 15 mph to 17.3 mph.
The U.S. Department of Transportation says travel times declined by as much as 34 percent after Syracuse, N.Y., optimized signal timing at 145 intersections.
Sitting in traffic costs time and money. A report released in February by the Texas A&M Transportation Institute found that we squandered an average of $818 each in 2011 while tied up in traffic.
The report said the total cost of wasted time and fuel to Americans was $121 billion — up $1 billion from 2010. It also concluded that clogged roads resulted in the release of 56 billion pounds of additional carbon dioxide into the atmosphere.
In the Twin Cities, the study found, congestion cost each commuter $695, 45th among metro areas.
Ryan Richter, an urban planner in Chicago who specializes in the relationship between land use and transportation, says most communities pursue traffic-signal synchronization as a way to increase capacity without having to widen roads.
In the short run, he says, delays can be reduced, especially during rush hours.
“Over time, however, studies have shown that as the road becomes more efficient, it enables induced demand,” Richter says. “The solution becomes a victim of its own success.”
Reduces energy consumption
The U.S. Department of Energy says on its website that traffic-signal synchronization increase efficiency, reduces energy consumption and eliminates or delays the need for street widening.
DOE also lists “potential disadvantages,” including increased travel speeds and additional traffic.
Reid Ewing, a professor of city and metropolitan planning at the University of Utah in Salt Lake City, says getting traffic-signal synchronization right is one of local governments’ most important responsibilities because it affects almost everyone.
It’s difficult, though, to get it right, he says:
- Too often, Ewing says, protected left-turn arrows slow the entire system.
- Cross streets can be given too much time on green lights, slowing traffic on major roads that intersect with them.
- Traffic engineers don’t always update signal intervals to match changes in volume and flow.
- To work correctly, signals must be close to each other or at least a quarter-mile apart. The progression of traffic through irregularly spaced synchronized signals isn’t optimal.
The best signal systems, which have been employed for years in Great Britain and Australia, is the one being installed in Minneapolis, Ewing says.
In the “adaptive timing” system, traffic is monitored in real time through “loops” or sensors in the roadway and signal intervals can be constantly adjusted.
More common in the United States, Ewing says, are pre-timed systems with signal timing locked in, and time-of-day timing that changes during peak and off-peak driving times.
Getting it right is vital not just to smooth traffic flow, but to ease drivers’ stress levels. Poorly timed signals, Ewing says, “grate on drivers, are irritating, a waste of time and a nuisance.”
Nozzi wishes city planners and traffic engineers would turn more often to alternatives to signal synchronization. He thinks traffic roundabouts are a good idea, though they “create intersections that are too large for pedestrians.”
He’d like to see streets made more bicycle-friendly and more residential development in downtowns so people don’t have to drive as much. And he suggests that streets be designed differently to force drivers to slow down.
“Motorists travel based on their perception of the fastest speed that they feel safe at,” Nozzi says. “The only way to really effectively slow down cars is to design streets to be more narrow and obligate the motorists to be more careful.”