Fighting Congestion in Minneapolis on a Tight Budget
In early October a power outage in Minneapolis threw the city's central traffic-signal control system into disarray. Lights at various intersections lost much of their coordination, with some lights quickly changing color, leading to intense frustration among drivers and not a few complaints. The confusion underscored the general antiquity of the city's traffic control system, which has been updated, but never overhauled, since it was first installed back in 1974.
The traffic situation in Minneapolis isn't pretty. The 2011 Urban Mobility Report ranked the Twin Cities third worst among large cities in annual delay per commuter, at about 45 total hours. Of course a number of factors contribute to city congestion, but the decades-old traffic control system in Minneapolis certainly isn't helping. Neither is the city's tight budget: in the recent past there has been enough money only to upgrade parts of the system piecemeal.
Over the next two years all that will change. With the help of several federal grants, Minneapolis will reconfigure its entire traffic system by 2013. The citywide effort will be the first universal upgrade to the signal system since the 1970s. The changes will be both technological and philosophical in nature: new timing patterns based on updated traffic data will be incorporated into all 800 light signals within the city, and a state-of-the-art central control system — capable of giving priority to transit modes like light rail — will lift the streets out of the disco days and into the 21st century.
For many commuters, the results will be as noticeable as the recent power outage, but in a positive sense. "I think overall we're looking at a 15 percent reduction in delays across the city," says Nickolas VanGunst, a traffic engineer with the city's Traffic and Parking Services Division. Achieving this improvement in flow won't break the city's bank either, coming at a total cost of roughly $5 million.
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There are three general parts to the city's traffic overhaul, says VanGunst. The first is the creation and implementation of sweeping new timing plans for intersections throughout the city. Broadly speaking, the effort involves a massive amount of data collection, so the city can consider current settings like cycle length and offset data (the amount of time signal B turns green after signal A) against the latest traffic volumes and land geometry. That information will help engineers model current traffic patterns and, by extension, improve signal coordination on high-volume arterial roads.
The timing plans will be subsidized by the federal Congestion Mitigation and Air Quality (CMAQ) Improvement program. According to one of the city's CMAQ grant applications, the updated timing patterns will reduce stops by 25 percent, fuel usage and delays by up to 15 percent, and overall travel time by up to 12 percent. They will also increase travel speeds and dramatically lower greenhouse gas emissions. A highly cited article on retiming traffic signals, published in a 2002 issue of Public Roads, estimated a benefit ratio of 40 to 1 for cities — much more cost-effective, say, than constructing roads.
One Minneapolis corridor long overdue for a timing update is Lake Street, a major commuter artery that runs east-west across the city, with dozens of traffic signals along the way. Timing patterns on the street haven't kept up with traffic levels or development in the corridor. During morning rush Lake Street experiences heavy congestion at 10 intersections, and in the evening that's the case at 28 signals, according to the CMAQ application.
Once congestion data is updated and a new plan is loaded into the central traffic system, travel flow will improve on arterial roads like Lake Street throughout the city. Right now, for instance, it's common for a car on Lake Street to clear only two or three lights before getting stopped. Ideally, says VanGunst, a car should be able to get through about 10 lights on Lake Street without stopping, and the new system will help achieve that level of movement.
"The central system is only as good as what you put into it," says VanGunst. "When we put in the new plans, and we modify this model to adjust for new plans, we can look at difference. In the future we'll have numbers we can look at and see how much delay we've saved people."
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The second component to the city's traffic plan is the central system itself. The city recently approved the purchase of an operating system called TACTICS, which is produced by Siemens. The advantages of TACTICS over the city's 1970s-era system seem well worth its $160,000 price tag. For one thing, says VanGunst, it will ensure that all the city signals have the same time reference. Right now the city's 800 signals have 800 different clocks. If one is at midnight and another at 12:01, coordination for an entire corridor can go awry.
The biggest distinguishing aspect of TACTICS is its ability to provide what VanGunst calls "transit-signal priority." In brief, the feature will enable Minneapolis traffic signals to detect buses, light-rain trains, or other forms of public transportation that approach an intersection. The signal and the transit mode then enter a dialogue about timing. If the bus is running behind schedule, the signal will explore its options for accommodating the bus within the current light cycle — for instance, it might add a few seconds to a green light on the main street so the bus can get through, and subtract a few seconds from a cross-street to make up the time.
What TACTICS does better than any other traffic system, says VanGunst, is maintain a general level of coordination among adjacent signals. Some transit-priority systems might take two or three light cycles to get back to baseline operations after a maneuver such as the one described above. That means general drivers are waiting at lights longer than they should be in some situations. TACTICS makes precise adjustments on the fly to keep traffic flow as close to the optimal settings as possible.
The transit priority is especially important to the city's burgeoning light-rail line. Currently the line runs from suburban Bloomington to downtown Minneapolis — connecting the Mall of America, Minneapolis-St. Paul International Airport, and the Metrodome in the process. Earlier this year complete funding was secured for an 11-mile extension of the system, known as the Central Corridor, that will connect Minneapolis with downtown St. Paul. The line is scheduled to open in 2014, and officials project roughly 40,000 riders by 2030. Though the light rail has its own right-of-way, proper coordination with the signal system is imperative to the mode's success.
"Transit, at least around here, is becoming much bigger, just because of the congestion," VanGunst says. "You can expand, but to what cost do you add another lane to a freeway? It's not worth it."
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The third part of the city's traffic project is completing its modernization of the physical signal controllers. Each traffic signal has a corresponding controller that essentially tells it what to do. Budget constraints have prevented the city from replacing more than about 10 controllers per year in the recent past. As a result there are still nearly 150 outdated controllers in operation. Federal funding will help the city replace within the next two years what would have taken nearly 15 years to complete otherwise.
While the old controllers can handle only basic timing patterns, new ones can store upwards of 64 timing plans at once; it's the difference between a film in black-and-white and one in Technicolor. The modern controllers, in conjunction with the modern operating system, will give the city's traffic engineers more options for dealing with traffic flow. Right now, for instance, most of the signals in Minneapolis are programmed to fixed timing patterns. Important roads will stay green longer than secondary cross-streets, especially during rush hour, but generally speaking the lights follow a rigid schedule.
The new controllers will make it easier for engineers to created actuated signals — the technical term for signals that can detect waiting traffic, and change their timing patterns accordingly. The new controllers will also improve intersection safety, says VanGunst, because they come with monitors programmed to detect conflicting movement signals. If one light is green and an opposing light wants to become green, for instance, the signals will recognize the conflict and immediately go into a flashing mode until someone can address the problem. The controllers can even preempt timing commands to accommodate emergency vehicles.
In the end, all three elements — the new timing plans, the cutting-edge central system, and the latest signal equipment — will work together to make the city easier to navigate.
"You take that signal timing plan and you download it to that controller, then you let it run its thing. The central system will poll that controller, either a minute or every five minutes, to make sure it's doing its thing," says VanGunst. "That's the system we're looking at moving to in the next two years."
Image courtesy Flickr user Drew Geraets