Safe, smooth, and long lasting are attributes of pavements that appeal to owners and users alike and are also commonly promoted benefits of pavement preservation. But how well do the pavement performance measures commonly collected by roadway agencies allow us to track preservation benefits such as these? Here’s a quick look at safety, smoothness, and pavement life, and how the desired properties can be measured.
Safety consistently ranks at or near the top of the list of attributes the public wants from their roads. If we focus on the surface characteristics in particular (and not other important roadway factors such as geometrics and the presence and visibility of signs and markings), measuring skid resistance or friction provides a direct indication of how quickly a vehicle can stop, especially under adverse conditions. Unfortunately, many agencies do not consistently monitor pavement friction, so it is difficult to document improved safety as an outcome of pavement preservation. An indirect measure of safety is the incidence of wet weather crashes, especially when a crash can be primarily attributed to surface conditions. While that information is more widely available, it is only generated after an incident.
After safety, smooth roads are arguably the second-most desired roadway attribute. A common measure of smoothness is the International Roughness Index (IRI), measured at traffic speeds using a high-speed profiler. Most state agencies and many local agencies regularly monitor the IRI to track the performance of their roadways.
Roads that last a long time are desirable to both the traveling public, as users and taxpayers, and to the owning agency responsible for managing them. There are a number of ways to track pavement life, such as the number of years from construction until major rehabilitation or reconstruction, or the time between preservation treatments. Treatment life may also be of interest, and one way of defining that is the time between the performance value that triggered the application of the preservation treatment and when the pavement reaches that trigger value again.
If roadway safety, smoothness, and long life are benefits expected from a pavement preservation program, here are some recommendations regarding a performance monitoring program.
First and foremost, it is important to know where preservation treatments are placed. Ideally, treatment location information is embedded within a pavement management system, accompanied by pavement history and performance data. Useful performance measures serve little purpose if not accompanied by tracking the location of preservation treatments. Equally important is knowing whether treatments are placed on good pavement for preservation purposes or on bad pavement as a stopgap measure; the same treatment does not perform the same under these vastly different conditions.
According to American Society for Testing and Materials (ASTM) E274, regular monitoring of skid resistance is the most proactive method of demonstrating improved safety from preservation. Of particular value is documenting before-and-after treatment measurements of friction to show how friction improved; regular measurements afterward will show how long the improvement lasted. Surface characteristics such as mean profile depth or mean texture depth are indirect ways to track the safety benefits of preservation.
An indirect gauge of roadway smoothness is the number of complaints an agency receives from the traveling public and anecdotally there are still agencies using complaints as a project selection tool. However, as noted above, regular monitoring of a roadway’s IRI (following ASTM E1926, for example) is an excellent, objective way of reporting roadway smoothness and demonstrating one of the key benefits of pavement preservation. A noisier ride, which may accompany the surface texture of some preservation treatments, should not be confused with increased roughness.
As noted above, a measure of years is needed to document treatment and pavement life. There is no single measure that is best for this purpose, and several suggestions for what to measure have been offered.
Revisiting those desired pavement attributes — safe, smooth, and long-lasting — it’s easy to make these claims about preservation and not that difficult to identify associated performance measures. However, in many instances pavement preservation competes against other approaches to managing a pavement network, and what may really be needed is to demonstrate that pavement preservation leads to safer, smoother, and longer-lasting roads. That’s an analysis that requires tracking where preservation treatments are applied and monitoring their performance but also comparing preservation performance to alternatives, such as untreated sections or sections treated with techniques other than preservation. Road agencies typically do this through data available in their pavement management systems.
David Peshkin, a Principal at Applied Pavement Technology, Inc., joined the firm in 1996. Since then he has focused his technical efforts in: preventive maintenance and preservation; pavement evaluation and design; technology transfer for pavement design, maintenance, and rehabilitation; and pavement research. Over the course of a more than 36-year career he has provided consulting engineering services on airport and roadway projects to a wide range of clients across the United States and around the world.
Written by David Peshkin, P.E.
Aug 21, 2023
