For years traffic data has been cars and clocks. How many vehicles and how long it takes to travel on certain roads. Now, planners want to think of traffic data in a different way.
Rock Hill-Fort Mill Area Transportation Study officials recently shared the latest figures about road reliability, based on new federal standards. It compares average travel time on main roads to peak times during heavy traffic.
“What we’ve really focused in on here is the difference between the two figures, the buffer,” said Chris Herrmann, RFATS transportation planner.
The idea is, even roads where it takes a while to go far can function if drivers know how much time it could take.
“They can plan for average delays,” Herrmann said.
Road reliability accounts for how often average delays become lengthier ones.
“This is the range of uncertainty drivers experience,” said David Hooper, RFATS administrator.
How it works
Major roads from interstates to federal or state highways have data for morning and afternoon. The federal standard measures peaks at 6-10 a.m. and 4-8 p.m. The RFATS data shows the average time it takes a vehicle to move on a road. It also measures the peak time as 80% of the worst traffic the road sees.
By using the 80% mark, it takes out one-time or rare traffic events that could skew the numbers.
A road as large as Interstate 77 receives road reliability scores in small sections. Northbound morning traffic reliability scores are worse than the federal standard from the S.C. 160 interchange north and at the I-485 traffic merge where I-77 loses a traffic lane.
Sections of northbound I-77 south of the S.C. 160 interchange score within the federal standard.
Reliability also factors into public perception of traffic. Someone expecting a drive to take 30 minutes will recall the few times it takes twice that long.
“Experiences like that stand out much more to the everyday driver,” Herrmann said.
As the area continues to grow, road reliability could factor into decisions from mass transit or road improvement planning to when residents leave their homes each morning.
“Limiting variation and improving reliability will become more and more crucial,” Herrmann said. “It’ll mean the difference for a parent trying to drop off their kids at school before the tardy bell rings, the difference in someone trying to get to work on time or the difference in someone trying to make that important doctor’s appointment.”
How the roads fare
Among major roads, the least reliable route is southbound U.S. 21 in the afternoon. The difference between average and near peak traffic travel is more than 11 minutes. It’s almost twice as long as the closest major roads.
Roads that score more than six minutes between average and near peak travel include northbound I-77 and U.S. 21 in the mornings, and S.C. 160 east of I-77 (eastbound and westbound) in the afternoon.
The most reliable routes are southbound I-77 in the morning and northbound I-77 in the afternoon (less than a minute difference each), I-77 southbound in the afternoon, westbound Celanese Road in the morning, westbound S.C. 160 west of I-77 in the morning and southbound Cherry Road in the morning.
Some major roads have seen reliability improve the past four years. Others have seen minutes added. They’ve largely seen traffic volume increases in that span, up to 35%.
York County Council Chairman Michael Johnson, who also sits on the RFATS policy committee, questioned some of the numbers. The average travel time on S.C. 160 in his area showed about seven minutes, which Johnson said he couldn’t hit on a Saturday.
“I live in Tega Cay,” he said. “It takes me 25 to 30 minutes every morning, and my office is right off the interstate.”
Herrmann said the longest recorded travel time in that area was about 40 minutes. That road highlights the difficulty in nailing down road reliability.
“It’s an imperfect reflection of what’s going on,” Hooper said.
The federal mark, for instance, looks at the morning peak at 6-10 a.m. But if half or more residents get on the road between 6:30 and 7:30 a.m., they’ll experience something different than drivers at 9:45 a.m.
“It’s that hour-and-a-half that they’ll remember, and it’s far beyond this level,” Hooper said.
Another issue is lane use. The data looks at major roads in whole. There are times cars backed up to get off southbound I-77 at Carowinds may stretch a mile back, while cars cruise through the left lane. It’s a similar story tracking drive times on Celanese Road in Rock Hill.
“That depends on what lane you’re in,” Hooper said. “If you’re in the far right lane from India Hook, it’s backed up all the way and you’re pushing along like you are at (S.C.) 160 and Gold Hill to get on the interstate. The data doesn’t capture that.”
The result is a set of numbers that only tells so much.
“The numbers are probably understating what the actual experience feels like,” Hooper said.
RFATS leaders say they can narrow data to more precise intervals. They can try to account for lane differences. The main reasons for looking at road reliability is to make the transportation experience smoother, whether by making road improvements or making a case for new transit options.
“It sends a pretty clear message about what needs to happen,” Hooper said of growth making roads less reliable, “in terms of land-use decisions and in terms of transit.”