A H Parham

DRIVER UNDERSTANDING OF PAVEMENT MARKING COLORS AND PATTERNS

Driver understanding of the current U.S. system of yellow–white pavement markings was assessed through a driver survey. The survey was used to evaluate drivers’ ability to describe the pavement marking color code, drivers’ reliance on pavement marking patterns when interpreting marking messages, and drivers’ reliance on pavement marking color when interpreting marking messages. Researchers surveyed 851 drivers in 5 states, with respondents representing 47 states, the District of Columbia, and Puerto Rico. The survey results indicate that drivers tend to use signs and other traffic as the primary cue to determine whether a road is one-way or two-way. A substantial proportion of respondents had an understanding of the use of marking color to differentiate between one-way and two-way roads, but this knowledge is not the primary tool that drivers use to distinguish the direction of travel on a road. Approximately 75% of the drivers surveyed understood the basic concept that a single broken yellow line separates opposing traffic on a two-lane road. The presence of a solid line (either double solid or solid and broken) in the centerline increases comprehension of directional flow to approximately 85%; more than 90% of the drivers surveyed understood that a solid line (either double solid or solid and broken) prohibits passing. Almost 95% of drivers indicated that passing is permitted with a broken line. The survey results indicate that the yellow–white pavement marking system is better understood than previously believed.

DRIVER UNDERSTANDING OF POTENTIAL ALL-WHITE PAVEMENT MARKING PATTERNS

Over the last decade, increasing attention has been devoted to the potential of converting U.S. pavement markings to an all-white system. There are many reasons and potential benefits for doing so. As part of an NCHRP study, researchers conducted many activities to evaluate the feasibility of implementing an all-white marking system in the United States. The research study findings on marking practices in other countries and the results of the driver survey on all-white markings are summarized here. The researchers found that 17 of the 22 countries contacted use an all-white marking system. However, numerous differences exist between the systems used in various countries, so there is no single all-white system on which the United States could base implementation. The survey findings indicate that the all-white markings evaluated in the survey did not have any higher comprehension levels than the current system of yellow-white markings. Therefore, implementation of all-white markings would need to be accompanied by an extensive driver education program. The survey findings indicate that the presence of a solid line as part of the centerline increases understanding of the two-way message of a centerline to about 85%. In developing the survey, the researchers determined that the use of various marking widths or stripe-gap ratios was not effective in conveying messages to drivers. On the basis of the study findings, the researchers recommended an all-white system not be implemented in the United States at this time.

Evaluation of Zero-Length Vertical Curves

To evaluate the use of zero-length vertical curves with respect to Texas Department of Transportation design practice, construction results, and vehicle dynamics, and to compare zero-length vertical curves with minimum-length design vertical curves, 20 zero-length vertical curves and 15 minimum-length vertical curves were evaluated. Evaluations performed included examinations of sight distance for zero-length vertical curves, surveying roadway profiles, and measuring vertical accelerations. Sight distance criteria were found to be generally inapplicable below 2 percent for sag vertical curves. For crest vertical curves, sight distance criteria were inapplicable below 1 percent for design speeds of 90 km/h or less and inapplicable below 0.5 percent for design speeds of 100 to 120 km/h. The results of vertical acceleration testing showed that, below 0.5 percent grade change, no practical difference was found between zero-length and minimum-design-length vertical curves. Between 1.0 and 0.5 percent grade change, significantly higher accelerations were measured for high-speed tests on zero-length vertical curves. Finally, it was found that zero-length vertical curves were more likely to meet drainage grade requirements than minimum-design-length vertical curves were.