One of the major emerging technologies in highway safety has been the improvement of the types of retroreflectivity sheeting used on overhead signs on roadways. Modern signs are very efficient at returning light back to the driver, creating a much safer nighttime driving environment. Because of this, many departments of transportation are opting to remove sign lighting, particularly in dark, rural areas. However, no guidelines currently exist that provide guidance in determining when to use lights.
The Texas A&M Transportation Institute (TTI) recently partnered with the Virginia Tech Transportation Institute (VTTI) to develop guidelines for providing effective nighttime performance of overhead signs.
“In the past 2–3 decades, the retroreflective sheeting materials that are used for guide signs have become so efficient that states are now pretty much satisfied with turning off sign lights, at least in the rural dark areas where there are no other competing light sources,” explained TTI Senior Research Engineer Paul Carlson. “However in dense urban areas with many competing light sources, traffic engineers still feel like signs need to be lit. The problem is that all this is on gut feel, and there’s not a lot of science to say ‘here’s a place draw the line where lights are needed and lights are not needed.’”
In order to develop the guidelines, the research team performed three tasks.
The first was to develop an image processing technique that simulated the way people felt about the visual complexity of a scene, or in other words, how quickly they could read a sign given a certain set of conditions.
The researchers then used VTTI’s smart road facility for a closed course test and investigated the legibility distances of three different sign legends and background configurations under different sign lighting treatments.
The third task was an open road study that investigated the effects of both sign luminance and visual complexity on the distance at which a driver can read overhead signs during a recognition task. “We used mobile photometric equipment and measured the brightness of guide signs in different areas,” said Carlson. “We wanted to create a scale from a low visual complexity scene to a high visual complexity scene using five different ratings. We recruited people to drive the corridors where we measured the signs and had the participants read the signs.”
Using the three results, the researchers were able to come up with new guidelines for American Association of State Highway and Transportation Officials (AASHTO) for sign lighting performance.
“The guidelines are based on a visual complexity rating system,” said Carlson. “A sign using modern retroreflective sheeting on a dark rural road received a rating of one, while a worst-case scenario sign, say in downtown Houston or Miami, would receive a rating of five. If you are in level five, it doesn’t matter what sign material you use, you still have to use light.”
The proposed guidelines are currently under review by AASHTO and will be added to their Roadway Lighting Design Guide.
NCHRP 828 | Guidelines for Nighttime Visibility of Overhead Signs: http://www.trb.org/Publications/Blurbs/174251.aspx