Section 3: Driver Needs
Anchor: #i999867Introduction
Requirements for pavement marking visibility are not based on characteristics of the roadway or the roadway geometry, but rather on the driver’s need for visual information in order to maneuver safely and effectively. To be effective, pavement markings must:
- present the appropriate visual clues far enough in advance of a given situation to allow for suitable reaction time to occur
- be visible in the periphery to aid in moment-to-moment lane navigation.
For visibility at night in a dark environment, the amount of light returned by pavement markings must meet or exceed these requirements. The amount of light returned is largely dependent on the retroreflective properties of pavement markings. But how much retroreflectivity is necessary for drivers to safely navigate at night? This question has been the focus of numerous research projects and has proven to be difficult to answer, partially due to the numerous factors involved with such an analysis.
Anchor: #i999892Factors Related to Driver Retroreflectivity Needs
Factors that influence the amount of retroreflectivity necessary for a given driver to navigate a given roadway at night may be categorized as either driver related or roadway related. Some of these factors include:
- driver related (each of which
often decline with age):
- visual capabilities
- cognitive processing capabilities
- motor skills
- roadway related:
- speed of the vehicle
- presence of continuous roadway lighting
- presence of retroreflective raised pavement markers.
Not all drivers need the same amount of light from pavement markings to safely navigate. For example, older drivers or drivers with visual impairments often need more light to see the same distance as a younger driver. Cognitive capabilities, which include attention and information processing, also decline with age. Cognitive declines often result in drivers’ having longer reaction times and increased driver workload. Declines in motor skills increase the amount of time needed to react to stimuli and perform driving maneuvers.
Similarly, roadway characteristics influence the amount of light needed. The speed of the vehicle influences the amount of light needed because at higher speeds a greater distance is needed to make a maneuver, thereby requiring earlier detection. Roadway lighting and retroreflective raised pavement markers both aid in the navigation tasks and reduce the amount of light needed from the pavement markings for safe navigation.
Anchor: #i999952Suggestions from Research Literature
Human factors research investigating the retroreflectivity needs of drivers includes three main types of evaluations: subjective evaluations, objective evaluations, and visibility models:
- Subjective Evaluations include studies where drivers are asked to evaluate pavement markings while they navigate through a roadway course.
- Objective Evaluations include studies where drivers are asked to detect pavement markings of varying retroreflectivity levels while driving through a course.
- Visibility Models are computer software packages used to predict necessary levels of retroreflectivity based on actual data.
The research literature contains numerous recommendations for minimum levels of retroreflectivity. When measured at 30 meter geometry, these minimum levels found in the literature fall in the approximate range of 80–620 mcd/m2/lx for high-speed roadways in a dark environment, with the range of 100–150 mcd/m2/lx encompassing most of the recommendations. Most of the subjective evaluations produced minimum values in the lower range, while the objective evaluations and modeling analyses suggested much higher minimum values. The research has aided the FHWA in development of recommended minimum in-service values for pavement marking retroreflectivity.