U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
In the United States, most longitudinal markings are nominally 4 inches wide. The MUTCD defines the width of a normal pavement marking as 4 to 6 inches, and a wide pavement marking as at least twice the width of a normal marking (at least 8 inches). In practice, however, many agencies consider 6-inch wide longitudinal pavement markings to be wide markings. As demonstrated in this chapter, much of the research on pavement marking width also considers 6-inch pavement markings to be wide.
A 2013 study published by FHWA includes a literature review of the impacts of pavement marking width on observed driver speed and lateral placement; however, the findings of the review were inconclusive.(6) The same FHWA report includes a before-and-after study of vehicle speed and vehicle lateral placement approaching and throughout horizontal curves on two-lane highways. The study produced findings similar to the previous research. While some particular instances of either lateral placement and/or change in speed were found to be statistically significant, the findings were not consistent and the magnitude of the change was not deemed practical. For the conditions studied, it appears that wider edge lines had no practical impact in terms of vehicle lateral placement and speed.
A motorist operating a vehicle uses pavement markings to guide him or her along the roadway. It is generally believed that, to be effective, pavement markings must be easily seen–increasing the width of the markings is one way to increase their visibility. Some agencies have reported that the visual appearance of wider pavement markings is enough to justify their use because the roadway looks better and even safer compared to 4-inch markings.(7) In general, research provides some evidence to suggest that drivers can see wider markings at longer distances. These improvements may improve lane keeping and positively impact safety.
Researchers have performed multiple studies to evaluate pavement marking visibility as it relates to width. The results of pavement marking width studies from the 1980s and early 1990s are inconclusive in terms of identifying visibility gains. Some research showed increased visibility for wider lines,(8,9,10) while other research showed no consistently statistical or practical differences.(11,12) More recent studies have provided more consistent results.
A 1995 study found that increasing pavement markings from 4 inches to 8 inches provides a statistically significant increase in average detection distances for young drivers on a left curve.(8) A 2001 study found that 6-inch markings have a statistically significant improvement over 4-inch markings for detection distances among both older and younger drivers under dry conditions at night.(11) A 1996 simulator study found that 8-inch markings provide a marginal improvement in average detection distance over 4-inch markings for both older and younger drivers at low levels of marking brightness.(14)
A 2006 study found that increasing markings from 4 inches to 6 inches resulted in an increase in detection distance, but found no increase in detection distance when increasing width from 6 inches to 8 inches.(13) A 2010 study comparing various 4 inch and 6 inch markings under wet and dry conditions found that some markings can produce marginal improvements in detection distances under wet conditions.(15) The findings show that the structural design of the marking, combined with the type of retroreflective optics, is an important aspect to understand in terms of the relationship between width and visual nighttime performance under both dry and wet conditions. A 2006 empirical study showed that theoretical calculations of marking detection distance as a function of marking width are invalid, and more work is needed to develop the mathematical relationships between marking width and detection distances.(20)
A 2010 eye-tracking study suggests that increasing edge line width from 4 to 6 to 8 inches along horizontal curves provides a reduced driver workload in the driving environment by providing more time for drivers to focus their foveal (central) vision on critical driving tasks.(16) The same study also determined that brighter pavement markings did not impact driver eye-looking patterns. Combined, these findings suggest that increasing the width of pavement markings may be more valuable than increasing their retroreflective performance–at least above a particular retroreflective threshold that may be adequate.
Being perhaps the most important performance indicator of the effect of pavement marking width, safety evaluations have been reported for nearly two decades. In general, as more data are available and more advanced study techniques are implemented, the effect of pavement marking width is better understood; however, the effect of increasing pavement marking width by 2 inches has been elusive. Some agencies have reported that the visual appearance of wider pavement markings is enough to justify their use because the roadway looks safer.(7)
In 1987, a study from Virginia evaluated wider edge lines using a before-and-after approach with run-off-the-road and opposite-direction crashes. The data were from three years prior to installing 8-inch wide edge lines and two years after installation at three test sections.(17,18) The analysis resulted in a 13.6 percent reduction in both run-off-the-road and opposite-direction crashes, which was not statistically significant when compared to the comparison sites. Another 1987 before-and-after crash study conducted in New Mexico suggested that wider lines have no safety benefit in terms of reducing crashes.(15) Both of these studies were hampered by insufficient data.
In 2012, researchers presented the most comprehensive analysis yet of wider pavement markings and their impacts on the frequency and severity of crashes.(29) The analysis excluded intersection and interchange crashes as well as those in the winter months of November through March. Researchers studied the data from two-lane highways from three different States using current statistical analysis techniques to handle the unique characteristics of the data, including differences of how, when, and the extent to which States made the transition to wider lines. Also adding to the challenge was the implementation process and timing of transitioning to wider edge lines in each State.
This detailed approach provided a number of results and while there are minor differences among the results, they all indicate consistent and positive safety effects of wider edge lines on two-lane highways. The crash frequency analysis suggests that wider edge lines are effective in reducing the frequency of crashes on rural two-lane highways, especially with regard to relevant target crashes such as single-vehicle crashes and run-off-the-road crashes.(6)
The same study also tried to identify the safety benefits of pavement markings wider than 4 inches on multilane divided highways. Using similar approaches but with less data, the research failed to produce the same conclusive findings. At the same time, the wider edge line markings were not shown to be a detriment either. One of the possible explanations is the standards to which these different types of roads are built. Since many two-lane highways have lower design standards than multilane facilities, it is not unreasonable to expect that subtle changes like adding 2 inches of pavement marking width will have a measurable impact on the lower standard highways. This is an interesting finding because agencies often prioritize the placement of wider pavement markings on multilane divided highways before their two-lane, two-way highways.
In a follow-up study, researchers developed and compared benefit-cost ratios for wide edge line markings and other low-cost safety countermeasures such as rumble strips and post-mounted delineators.(21) Using earlier research findings,(29) specifically the conclusion that wider edge lines have been shown to reduce total target crashes 15 to 30 percent and fatal plus injury target crashes 15 to 38 percent, the researchers estimated that the benefit-cost ratio for wide edge lines is $33 to $55 for each $1 spent. Coincidentally, this benefit-cost ratio is similar to the benefit-cost ratio for rumble strips alone, although there are obvious differences between the two countermeasures. While rumble strips address crashes where the driver is distracted, drowsy, or inattentive and can be effective even when obscured by snow or rain, wider edge lines seem to be most effective where the driver is looking at the roadway/striping, or where the driver's peripheral vision is detecting the marking.(22)