The Ohio DOT (ODOT) has specified longitudinal diamond grooving for concrete bridge decks, both new construction and overlays, since 2012. Cutting narrow grooves into pavement reduces hydroplaning risk, increases drainage at the tire/surface interface, and improves vehicle control. Statistics show a clear relationship between longitudinally grooved surfaces and a reduction in accident rates.
However, only 2% to 3% of state roads are concrete. Asphalt over brick or concrete comprises 45% to 48%; the rest are full-depth asphalt. The performance of longitudinal grooves on concrete bridge decks encouraged the agency to try the treatment on asphalt curves.
"It's an out-of-the-box solution," says Office of Technical Services Highway Infrastructure Management Supervisor Brian Schleppi.
At one time, pavement engineers believed transverse tining/grooving provides better friction and water removal than longitudinal grooving. While it seems channels cut perpendicular to traffic lanes would drain faster, data don’t support that assumption. Most studies are laboratory-based or emphasize groove over direction. In the 1960s and early 1970s, Caltrans compared 322 lane miles of longitudinally grooved concrete to 750 miles of ungrooved concrete on road sections with average daily traffic of 60,000 to 200,000 vehicles. Accidents fell an average of 70% on the grooved pavement; 2% on ungrooved sections. The agency concluded longitudinal grooves reduce accidents, regardless of weather, by an average of 69%.
Why? Tire treads embed in the grooves and are then able to resist the vehicle’s lateral motion, which enhances stability. In addition, transverse grooves often terminate before a free-flow condition out of the groove. This happens when pavement stripes block flow or the groove doesn’t extend to an open-flow condition. In such situations, which aren’t uncommon, transverse grooves merely provide additional water storage capacity.
"It's easy to get stuck on the idea of moving water off to the side of the road, but it's best to use gravity to move most of it off and then displace the small amount that remains,” says Schleppi. “On a curve, longitudinal grooves keep cars in their lane. This is due to mechanical bite: Tire tread blocks have a lateral edge that’s forced into the groove. Once that happens, significant force is required to lift tread blocks out of grooves."
A gentler way to improve friction
Rain and snow compromise friction on two levels:
- Microtexture: surface aggregate, where tire adhesion occurs
- Macrotexture: the larger surface over which water is evacuated.
However, you don't need the same friction level everywhere. “We talk about ‘friction demand,’” says Schleppi. Lower speed limits, for example, typically require less friction. The problem in sections with above-accident rates may not be inadequate friction but something else, like sight distance or roadway configuration and geometry. In such cases, improving friction may mitigate these issues.
Ohio addresses friction by mechanically abrading the surface (grinding, grooving, micromilling, shotblasting, etc.) or covering the surface (using a high-friction surface treatment [HFST], overlay, microsurfacing, etc.). Overlays cost more, but pavement engineers also consider skid test results, surface conditions, longevity, cost, and project duration when deciding which to use.
“HFST may be justified in highest-friction-demand locations; in other areas we can realize the same benefits by using lower-cost treatments such as longitudinal grooving or diamond grinding,” Schleppi says. Unlike carbide mill heads that use force to remove material, diamond saw blades gently abrade the surface. Unlike overlays, grooving can be done year-round, even in low (although not freezing) temperatures. In addition to enhancing friction, the treatment improves ride quality and is less noisy than other surface treatments.
Of the $102 million ODOT spends every year on safety-related projects, $10 million to $20 million goes toward systematic improvements: minimally invasive, cost-effective tweaks quickly deployed over many miles. National studies show that things like median cable barriers, edge line rumble stripes on high-speed rural roads, and reflective backplates on traffic signals at busy intersections reduce crashes.
In 2013, a curve signage systematic safety program required the agency’s 12 districts to take an in-depth look at crashes occurring at certain locations. The agency choose three and contracted BOCA Construction Inc. of Norwalk, Ohio, to groove them. Performance is still being monitored, but results are encouraging. Post-project testing per ASTM E524 – 08 (2015): Standard Specification for Standard Smooth Tire for Pavement Skid-Resistance Tests, which measures friction in a forward direction with no lateral force, show improvement in skid numbers (SN). More importantly, crash rates are down.
Writing asphalt-grooving specifications
Commercial trucks were experiencing higher-than-expected crash rates while navigating a heavily traveled, high-speed, multiple-curved section of I-75 in Montgomery County, Ohio. Skid testing revealed good microtexture, but macrotexture required improvement.
In June 2013, after rejecting high-friction surface treatment (HFST) as cost-prohibitive, the Ohio DOT decided to longitudinally groove about two lane miles. The agency’s bridge deck grooving specifications were written for concrete, so District 7 Engineer Scott LeBlanc modified them for use on asphalt.
"The specification was developed for a rigid structure but asphalt is inherently more flexible,” he says. “We wanted to avoid creating tall, slender columns of asphalt that might compress into the groove or fatigue and break off." As a result, grooves are slightly wider and, to account for the likelihood that asphalt would compress into the valleys of the grooves and reduce their effective depth, slightly deeper.
Bridge deck specifications require grooves to be 0.75 inches apart (minus 0.25 inch to plus 0 inch), 0.10 inches wide, and approximately 0.15 inches deep. Grooves in asphalt are also 0.75 inches apart, but at minus 0 inch to plus 0.125 inch to provide a slightly wider pattern; 0.125 inches wide; and 0.25 inches deep (minus 0.1 inch to plus 0 inch). These dimensions and tolerances ensure water is evacuated from under tires while taking into account asphalt’s flexibility to avoid creating overly slender columns.
Data show the grooving virtually eliminated crashes on northbound lanes. Accidents on southbound lanes are down, but ODOT may pursue road geometry changes in the future. In the meantime, District 7 conducts one to two skid tests per year to measure friction and project the asphalt’s remaining life. The grooved section improved average SN 40 smooth tire value by more than 70%, 1.7 times its previous value.
Milling vs. grooving: real-word comparison
The I-90 innerbelt near downtown Cleveland was supposed to connect to a road built by the Works Progress Administration via a future freeway, but that never happened. Despite speed limit reductions, a nearly 90-degree turn has two to three times more accidents than the regional average. Some involve fuel spills, which softens asphalt.
The agency uses Superpave asphalt mix design, a specification that categorizes binders into temperature- and climate-based performance grades and requires aggregates with specific angularity and gradation requirements. It’s also based on volumetric mix design and gyratory compaction in the lab that relates compaction effort during mix design to expected traffic.
In 2014, in an attempt to lower the accident rate, the agency chose a 12.5-mm Superpave mix with a higher-grade binder and a high-quality course aggregate known to improve friction. Skid testing for informational purposes (to begin evaluation of the mix) led to further investigation revealing the mix reduced macrotexture.
Schleppi proposed giving eastbound lanes a fine carbide mill treatment, a common solution for this project type, and westbound lanes longitudinal diamond grooving. A curve of less than a quarter-mile was longitudinally grooved in November 2015. Despite the tight radius, the cutting blades performed well while making the turn and sustained no damage.
Both surfaces are performing as well as, or better than, expected. Pre- and post-project data show wet weather crashes have decreased. Diamond grooving improved the average SN 40 mph smooth tire value by more than 50%, 1.5 times the previous value. ODOT is leaving the surfaces intact even though the curve’s part of a larger section of I-90 scheduled for preventative maintenance surface treatment this year.