Our Road Maintenance Division spends a fair amount of time maintaining gravel road surfaces, year-round for many of them. Usually this takes the form of grading (reshaping the surface) and gravelling (adding additional aggregate). If you’ve driven on various gravel roads, you know there can be quite a bit of difference from one to another. For example, different rock types are used to try and better accommodate the differences in roadway configurations, locations and traffic.
Common complaints about gravel roads usually revolve around potholes and ‘washboarding’ – those repetitive bumps that can really get a driver’s attention to slow down. Hopefully this information will help you (and your neighbors) treat your road right and help limit the need for ongoing maintenance. Additionally, we want to offer some safety tips.
Having driven on a gravel road almost every day for over 21 years and worked in the transportation maintenance industry for over 30, I hope to share my knowledge with you. As a resident on a county-maintained gravel road, I get it. I’ve dealt with potholes, sloppy spots, washboards, dust, and just about everything else you can encounter on a gravel road.
The condition of a gravel road is primarily affected by two factors - the speed of the traffic and the moisture content of the roadway surface. You may have noticed that the volume of traffic is missing from this equation – in short, if speed and moisture levels are just right, there can be thousands of trips per day on a gravel road and it will still look and drive great. So we can’t blame poor road surfaces on the amount of traffic on our roads, but we can blame the speed of those vehicles.
You’ve likely heard the saying ‘Speed Kills’. It just so happens that speed can ‘kill’ a smooth gravel road surface and tun it into a washboard mess, sometimes in a very short amount of time. The cause of washboard comes down to basic physics and natural movement of materials. Wheels rolling over gravel will ‘move’ the aggregate and it will begin to settle in a pattern of ripples; the faster the vehicle speeds, the more movement occurs.
As noted in a 2009 study by the Federal Highway Administration on gravel roads:
Vehicle speed greatly contributes to washboarding; according to Stephen Morris, lead physicist for the University of Toronto study, “The hopping of the wheel over the ripples turns out to be mathematically similar to skipping a stone over water.” He explains that just as “a skipping stone needs to go above a specific speed in order to develop enough force to be thrown off the surface of the water,” vehicles need to be moving at a certain speed in order for washboarding to occur. Places where drivers routinely accelerate rapidly or brake hard are also particularly prone to corrugation — so one way to reduce washboarding is to simply lower the speed.
Speeds less than 3 MPH tend not to generate washboard, but that is really slow and no one wants to drive that slow. Keeping speeds under 20 MPH will SIGNIFICANTLY slow the rate of washboard development. So slow down and enjoy the scenery and this will help your road surface remain smooth much longer. Sadly, many of our county gravel roads can see some cars exceeding 40 and even 50 miles per hour.
The moisture content of the roadway will also play a role. You may have noticed some gravel roads almost look paved during portions of the winter season; a result of occasional rainfall keeping roadways damp enough so aggregate remains rolled into the surface by driving on it. However, too much moisture and puddles occur and weaken the structure, creating ruts.
To maintain a smooth gravel road surface longer, here are some tips:
- Slow down – ideally to less than 20 MPH.
- Minimize acceleration and braking (these actions cause more material to move).
- Minimize drastic acceleration when climbing hills or traveling through/coming out of corners.
- Utilize 4-wheel drive (per your vehicle owner’s manual) to minimize tire slip when climbing steeper grades.
- Avoid hitting potholes – especially when they are full of standing water.
Regarding safety, here are some tips for driving on gravel roads:
- Be on the lookout for changing road conditions – gravel road surfaces can change very quickly compared to asphalt roads
- Slow down. Driving slower will not only extend the life of a smooth surface, but it will also ensure safer travel.
- Watch for bumps. Whether washboards or potholes, these bumps can cause your tires to lose contact with the road surface. Enough loss of contact and your car will simply continue in the direction of travel – which could be off the roadway.
- Braking distance can increase substantially under certain conditions. Slowing your speed will help keep you on the road and out of the ditch.
- Increase your following distance when conditions are dusty. Not only will the air filter on your vehicle last longer, but you’ll be able to see better – day or night.
- When washboards show up, don’t increase your speed to ‘skip over the bumps’ – this approach will reduce tire contact with the roadway and may result in a crash.
Similar to other maintenance work we do for our road system, your help is needed to minimize maintenance needs and cycles. For example, keeping debris such as leaves out of roadside ditches and curb inlets can help minimize plugged culverts and storm drain systems. Thank you in advance for your efforts to drive safer as well as maintaining smooth driving surfaces as long as possible. With limited resources to grade roads, especially in the drier seasons, anything you can do to help is appreciated.