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28/03/2012

Study proves road safety benefits of wider edge markings

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JON SPROUL
John Sproul

Road markings are among the most cost-effective solutions to make roads safer by giving orientation and avoiding accidents. A new study, carried out by the Texas Transportation Institute (TTI) for the American Glass Bead Manufacturer’s Association, quantifies by just how much and provides a cost-benefit analysis.

Despite only making up 23 per cent of the US mileage, fatalities on America’s rural two-lane highways made up 57 per cent of all traffic fatalities in 2009 — resulting in more than $77 billion in losses for that year alone. Moreover, a rural motorist is 2.7 times more likely to be involved in a fatal crash per mile travelled than their urban counterpart.

The TTI study, entitled An Evaluation of the Effectiveness of Wider Edge Line Pavement Markings, shows that, just by using six inch (15.25cm) line marking, instead of the still widely used four inch edge line, there is a dramatic improvement in safety and reduction of fatalities on rural two-lane highways. It says that the use of six inch edge lines on rural, two lane highways have been shown to reduce total crashes 15 to 30 per cent, and fatal plus injury crashes 15 to 38 per cent. In addition, the benefit-cost ratio for wide edge lines is $33 - $55 for each $1 spent

Other techniques

The study also looked at other road marking systems and assessed their safety and cost benefits, including rumble strips, chevrons, and raised retroreflective markers. It examines and cites various studies on each technique before providing a comparison for wider edge lines.

For instance, a recent analysis quoted found that rumble strips reduce fatal run-off-road (ROR) crashes by 29 per cent and injury ROR crashes by 29 per cent. This reduction will result in an ROI ranging from $46.00 for every $1.00 invested to $37.00 for every $1 invested, depending on the circumstances and type of treatment used.

In the case of chevrons used on curves, the TTI study cites research that found that the installation of new chevron signs at a curve can reduce fatal crashes by 16 per cent and provide a benefit-cost ratio range of $8.60-$45.90: $1. The range depends on the cost assumption for installation - between $160 and $30 per sign.

Meanwhile, raised retroreflective pavement markers (RRPMs) from another study found that RRPMs decrease all crashes by 54.3 per cent and result in a benefit-cost ratio of $13.16 for every $1 invested. TTI researchers pointed out that there are some minor differences between the methodology used in this study and the other cited studies. Most notably, this study selected a five per cent discount rate, versus the seven per cent rate used above. This difference (and some minor other differences) may reduce the accuracy of any comparison.

Comparison

At the TTI study states, each of these policies is a unique method to decrease crashes and make the roadways safer. They all have specific circumstances in which they can be best utilised, so it is inappropriate to say that one policy should be used at the exclusion of the others. Nonetheless, a comparison of the cost-benefit ratio of each policy can be of significant informational value.

The result of the analyses is that wider edge lines result in an ROI comparable with the alternative policies. These results provide strong evidence to support the installation of wider edge lines on rural, two-lane highways.

Recommendations

In its recommendations, the TTI points out that rural highways are unsafe when compared to other road types. They account for a disproportionately large number of injury and fatal crashes. There are a variety of policies that state DOTs can pursue to address this issue, but one of the most cost effective is increasing the width of edge lines on rural two-lane highways.

“Agencies can use the results herein to develop a policy for six inch edge lines on their rural, two lane highways. Wider edge lines have been shown to reduce total crashes 15 to 30 per cent, and fatal plus injury crashes 15 to 38 per cent. In addition, the benefit-cost ratio for wide edge lines is $33 to $55 for each $1 spent, which is similar to rumble strips. If an agency is considering installing rumble strips as a safety countermeasure but concerned about the potential noise or pushback from the bicycle community, wider edge lines appear to offer similar results but without the concerns often associated with shoulder rumble strips.”

Concluding thoughts

Many states have adopted wider edge lines on a variety of roads for a variety of reasons. The adoption has been slow and uncoordinated, without sound empirical findings to support their policy choice. Previous studies found indirect evidence to support the adoption of wider edge lines, but evidence showing direct reductions in crashes were difficult to provide due to data issues (lack of appropriate safety data, lack of enough miles of wide edge line treatments, and naïve analyses techniques). The recent analysis of data from three states provides the necessary evidence to support the adoption of wider edge lines on rural two-lane highways. Based on the results of the cost benefit analysis above, state DOTs can now have further reassurance that their actions are in the best economic interest of society as well.

In some states, wider edge lines are first employed on freeways and other high-speed divided highways. The higher-speed is typically used to justify the wider edge lines. The most recent crash analysis shows that wider edge lines may have not significant benefit on freeways and other high-speed divided highways. Perhaps the subtle change is overpowered by the already high design standards used for freeways and other high-speed divided highways.

Finally, since the safety benefits of wider edge lines are now better documented with rigorous statistical analyses, it would be beneficial if the MUTCD could be amended so that the minimum edge line width on rural two-lane highways was six inches. Agencies would then be able to implement the policy in a uniform and consistent manner.

Expert comment
Asked to comment on the TTI report, prepared on behalf of the American Glass Bead Manufacturer’s Association (AGBMA), Jon Sproul, general manager of glass bead and road marking material manufacturer Swarco Industries and a member of the association, told the Daily News that the TTI report was a major contribution towards improving safety on the roads.

"While previous studies always found that edge markings have a positive impact on traffic safety, no reliable statistical data was available to support the hypothesis that wider edge lines further improve traffic safety. The TTI report is based on recent statistically sound data and several surveys conducted over the past decade. The findings confirm that an increase of merely 2 inches in the width of edge lines will provide better guidance and visibility and hence reduce the number of crashes on rural two-lane highways," says Sproul and emphasises that road markings still remain among the most cost-effective and reliable solutions to make roads safer.

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