It's been six months since the state Department of Transportation turned on new traffic lights meant to moderate the flow of cars onto Interstate 540 in North Raleigh, and the NCDOT says so far they've helped increase average speeds and cut commute times on the highway.
But the data compiled by researchers at N.C. State University also show that traffic on the four roads leading to the ramps has not consistently improved, and in fact is sometimes worse.
Ramp signals have long been used in other parts of the country, but these are the first in North Carolina. The lights alternate between green and red to let one or two cars onto the freeway at a time. Putting some space between the vehicles leaving the ramp should make it easier for them to merge into traffic on the highway, improving the flow and preventing accidents.
Joe Beaman, an environmental consultant who lives off Falls of Neuse Road north of I-540, passes through the interchange every weekday, either to get on the highway to visit clients or to cross it to get to his office. Beaman says the backups on southbound Falls of Neuse appear longer since the lights were turned on in late September.
“There’s less congestion once you’re to the lights and you’re allowed to merge on to the interstate. It’s easier, because everyone’s not waiting until the last minute to merge in," he said. "But I can’t see how it’s alleviating the backups on Falls of Neuse.”
In addition to Falls of Neuse, the lights were installed on ramps onto westbound I-540 at Six Forks, Creedmoor and Leesville roads, and are used only during morning rush hour or other times of heavy traffic. For the first three months, NCDOT employees turned the system on and off at set times around the morning commute, usually 6:30 to 9 a.m.
But NCDOT started getting complaints when the lights were stopping cars even in light traffic. So starting in December, the department began letting the system turn itself on and off, using sensors in the pavement that determine whether traffic is heavy enough to need modulating, said John Sandor, the division traffic engineer. Since January, the computers have determined not only when the lights are on but also the length of time between red and green.
“Once we took the humans out of it, they started working a lot better," Sandor said.
The Institute for Transportation Research and Education at NCSU will provide a detailed report on the lights and their effects on traffic and accidents by late summer. In the meantime, the group's weekly reports to NCDOT indicate that overall there's been a reduction in the amount of time it takes westbound traffic to get from various points off I-540 to the U.S. 70 interchange, Sandor said.
Most of that improvement comes from faster travel on I-540 itself. During a week in mid-December, the average speed on the highway was 51 mph, compared to 35 mph during the same week a year earlier, according to NCDOT.
But factoring in the time it takes to approach the highway and get on, the results are often mixed. During the week of March 5, the NCSU researchers found that on nine routes onto the this stretch of highway, average travel times were 4.5 percent above the average for a six-month period before the lights were installed.
Sandor said the results seem to vary from week to week, but acknowledges that improving the travel times on I-540 may mean more snarls on the streets leading to it.
I’ve been honest with folks, you might see more delays on the feeder roads," he said. "It’s a balancing act that we’ve really learned as we’ve gone through this."
'Foolish' or 'commonsensical'?
Ramp signals, or ramp meters, are common in other large cities, including New York, Houston and Atlanta. They've been used on freeways in Chicago since the 1960s.
But as the first in North Carolina, the North Raleigh lights were met with some skepticism that their performance so far hasn't completely erased. Wayne Holley, an engineer who is studying law and lives off Gresham Lake Road, thought the lights sounded like a bad idea when he heard about them, and he still does.
“I’m headed somewhere, everybody on the entrance ramp with me is going in the same direction, and all of us are trying to accelerate to 540 speed," Holley said. "And those foolish stoplights prevent what people know how to do and is expected.”
But others are pleased that NCDOT is using technology to try to improve traffic flow. Jake Potter, who lives off Falls of Neuse and works in health care recruiting and marketing near PNC Arena, regularly encounters the lights during his commute and calls the system smart and "commonsensical."
“I’m not usually the first in line to pat the government on the back for being forward thinking, but to me this is a good innovation," Potter said. "It’s thoughtfully designed. I’ve noticed smoother traffic flow on most days.”
Sandor said the goal of the lights is not only to shave a few minutes off the morning drive to work for thousands of people, but also to make that commute more reliable, by cutting down on accidents that can bring traffic to a stop. The detailed NCSU report will compare the number of accidents along I-540 before and after the lights were installed.
NCDOT will use what it learns from the North Raleigh lights to determine where to install them along other highways in the Triangle and other cities. The department also says the lights will help make the most of three-lane I-540 before it becomes necessary to add another lane, at the cost of tens or hundreds of millions of dollars.
The light system cost $2.5 million to install on the four on-ramps, which included some widening of the ramps and the software that can be used as other ramp meters are introduced around the state.
“These things are so cheap compared to a road project," Sandor said. "If we didn’t do this, people would say, 'You guys are stupid for not trying this.' ”