Summary of Observations

Preparing Highways for Automated Driving Systems

Transportation Infrastructure Owner Operators (IOOs) are increasingly interested in understanding how they can prepare highways for automated driving systems (ADS). The primary reason for this interest is that ADS are projected to have dramatically positive impacts on highway safety while reducing congestion. Some estimates suggest that automated vehicles could reduce the world’s 1.2 million highway fatalities by 50 percent and save 250 million hours of commuting time.

While there will be decades of a “mixed-fleet” environment where both human drivers and machine drivers will be operating vehicles, the era of this “mixed-fleet” is just starting. Currently, 99.X percent of vehicles are driven exclusively by humans. The 0.X percent not human driven have partial automation features such as adaptive cruise control (ACC), lane departure warning (LDW), and lane keep assist (LKA). It will take time for the ratio of human to machine driven vehicles to shift drastically, but there is no doubt that it has started. Being able to align the progress of ADS with the pace of the highway industry will be a challenge, but it will also be a critical step towards preparing the nation’s highway system.

It is projected that by 2020, LDW technologies will be standard on 40-80% of new car sales, and that number increases to 70-99% by 2025. Similarly, by 2020 LKA technologies will be standard on 10-24% of new car sales, and 30-73% by 2025. LDW and LKA are intended to keep vehicles on the road and in their lane. They’ve been emphasized here because they address roadway departure crashes which are the largest category of crashes involving highway fatalities (approximately half of all highway fatalities). Roadway departure crashes as a result of distracted and/or impaired drivers are one of the most significant safety concerns that ADS features can positively impact. Despite the limited vehicles on the roadway with such technologies, research is already starting to prove out their benefits (1).

LDW and LKA technologies depend on well-maintained pavement markings. These technologies do not work as intended when the pavement markings fall below a certain condition. The National Cooperative Highway Research Program (NCHRP) has funded research to determine pavement markings characteristics that provide reliable machine vision detection. ATSSA recognized the value of that research and provided additional resources to evaluate four- versus six-inch pavement marking widths to confirm the improved reliability of machine vision systems and LDW and LKA features.

The research demonstrated that six-inch wide pavement markings consistently improve machine vision detection under adverse visibility conditions; and when combined with results from the pending NCHRP study, demonstrates that six-inch wide pavement markings can improve machine vision detection on high-speed roadways where potentially conflicting signals may confuse machine vision systems from detecting pavement markings. This includes areas with remnants of previously removed markings, pavement scarring due to removal activities, blackout markings, crack seal, longitudinal seams in the pavement, varying road surfaces, cracking, rutting, horizontal curves, or areas where glare is common and impacts marking visibility.

Recent research has also shown that six-inch wide pavement markings are good for human drivers too, making for an ideal infrastructure-based solution in the “mixed-fleet” era. In 2010, the Texas A&M Transportation Institute (TTI) published a multifaceted study evaluating the trade-off between increased pavement marking width versus increased retroreflectivity levels (2). A closed course study using metrics including vehicle lateral placement, speed, and lane-keeping glances showed that with increased pavement width, the likelihood of edge line encroachment decreased by 60 percent and the percentage of non-lane keeping glances also decreased.

In 2013, an FHWA-directed research project was published that included a focus on the safety effects of six-inch wide edge lines (3). This study included safety analyses of crash, roadway, and vehicle data from Michigan, Illinois, and Kansas. For two-lane, two-way roadways, the study showed that six-inch wide edge lines reduce fatal and injury crashes from 15 to 38 percent (4). As a result of this study, the FHWA Crash Modification Factor (CMF) Clearinghouse adopted CMFs for converting four-inch wide pavement markings to six-inch wide pavement markings (CMF=0.635 for all crash types in rural areas).

A crash severity analysis found that a reduction in the proportion of higher severity crashes was also associated with the six-inch wide edge line markings. Subsequent studies in Missouri and North Carolina have confirmed the benefits of six-inch wide pavement markings that were derived from Michigan, Illinois, and Kansas data.

In Conclusion…

The new ATSSA study has provided evidence demonstrating that six-inch wide pavement markings provide improved detection for machine vision systems. In addition, recent research shows that six inch wide pavement markings are also beneficial for human drivers in terms of lowering fatal and injury crashes on two-way, two-lane highways.

By 2025, most new car sales will include LDW features and approximately half will include LKA features. It seems clear that highway agencies can begin to prepare their roadways to maximize the safety benefits of ADS by marking their highways with six-inch wide pavement markings. The ATSSA study adds to the already compelling evidence showing that six-inch wide pavement markings will be good for machine vision systems and will also lower fatal and injury crashes associated with human-driven vehicles. The change in pavement marking width is “technology neutral” and provides a very broad and diverse societal impact. In 2018, California joined the national trend and increased their pavement markings width from four-inch to six-inch. Adopting a national policy for six-inch wide pavement markings helps address uniformity, provides a proven countermeasure to lower fatal and injury crashes, and prepares highways for ADS features.


  1. J. Cicchino. Effects of lane departure warning on police reported crash rates. Journal of Safety Research, 66, 2018, pp 61-70.
  2. J. D. Miles, P. J. Carlson, R. Eurek, J. Re and E. S. Park, “Evaluation of Potential Benefits of Wider and Brighter Edge Line Pavement Markings,” Texas Department of Transportation and the Federal Highway Administration, Austin, TX, 2010.
  3. J. D. Miles, P. J. Carlson, R. Eurek, J. Re and E. S. Park, “Evaluation of Potential Benefits of Wider and Brighter Edge Line Pavement Markings,” Texas Department of Transportation and the Federal Highway Administration, Austin, TX, 2010.
  4. E. S. Park, P. J. Carlson, R. J. Porter and C. K. Andersen, “Safety effects of wider edgelines on rural, two-lane highways,” Accident Analysis and Prevention, vol. 48, pp. 317-325, 2012.
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