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RESEARCH PAYS OFF
MICHIGAN DOT REFLECTS ON SIGNSDAVID LONG
he nighttime effectiveness of mosthighway signs depends on theirretroreflective sheeting, which reflects
vehicle headlights, allowing the text of thesigns to be read. As the sheeting ages,however, it becomes less effective at reflectinglight, and the legibility of the sign graduallydecreases. At some point the sign should bereplaced, but at what point? To date,transportation agencies have no standards toguide them in making this determination. Thissituation is about to be rectified. In 1993Congress directed the Secretary ofTransportation to develop retroreflectivitystandards for signs. The Michigan Departmentof Transportation has developed a new methodto assess retroreflectivity, thereby aidingcompliance with the new standards.
PROBLEM
Traditionally, a sign's retroreflectivity isassessed in one of two ways. The first methodis to rate signs visually on the basis of anobserver's assessment, either at night using avehicle's headlights or during the day using aQ-beam light source. The second method is toplace a retroreflectometer against the sign. Theinstrument emits a beam of light and thenmeasures the amount of reflected light.
Both methods have shortcomings. In visualrating, individual observers will score the samesign differently. Even the same observer willassess a sign differently as he or she becomestired. Comparing signs is difficult becausevariations in background and illumination levelaffect a viewer's assessment of a sign. Glarefrom the headlights of oncoming vehicles has adramatic effect on the perception of a sign atnight. Although the retroreflectometer is anobjective method, it can be expensive andtime-consuming. As many as 60 measurementsmay be needed to evaluate the retroreflectivityof a large sign; frequently a lane of traffic mustbe closed to do so.
SOLUTION
The Michigan Department of Transportation inconjunction with the Federal HighwayAdministration, has developed an innovativetechnique to measure a sign's retroreflectivityaccurately and economically. The technique isthe culmination of several research efforts,including National Cooperative HighwayResearch Program (NCHRP) Project 5—10, AMobile System for Measuring Retroreflectanceof Traffic Signs. This project yielded a systemconcept for rapidly assessing theretroreflectivity of signs and demonstrated theconcept's feasibility through the creation andtesting of a proof-of-concept model and theconstruction of a prototype system. In 1990this system was presented to NCHRP andturned over to FHWA for refinement anddissemination. Since that time, the agency hasbeen working with MDOT to develop themobile evaluation of traffic signs (METS)system.
The system consists of a van equipped withtwo video cameras, a flash tube, a lasermounted on the roof, and a computer and twovideo monitors inside the van. None of theequipment represents new technology, but theway it is put together makes the system theonly one of its kind. Using METS, a two-person team can evaluate 300 to 400 signs aday. The flash tube is sufficiently bright thatsigns need not be evaluated at night. BecauseMETS operates at highway speeds, the flow oftraffic is never disrupted.
As METS travels down the road, the videooperator keeps the video camera and laserfocused on the sign. The laser measures thedistance between the van and the sign beingevaluated. When the distance measures 62meters (203 feet), the flash tube illuminates thesign. A slight delay before activation of thecameras allows the whole sign to be capturedat 61 meters (200 feet), the optimal distance forthe camera lenses. After the image isTR
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captured, the computer digitizes and stores it asa 256-kilobyte file.
A software system developed by MDOT usesthe digital black-and-white image to calculatethe retroreflectivity of the sign. All of the pixelsin the continuous gray-scale image areconverted to either black or white. This image isthen used to calculate reflectivity values, aftercalibration for different sign colors. Separatevalues for the legend and background can becalculated, allowing the contrast ratio to bedetermined. The color image has a wider viewthan the black-and-white image and is used toidentify and inventory the signs.
APPLICATION
MDOT has used METS to assess the retrorefl-ectivity of 10,000 signs since 1994. The systemcan produce a report on an individual sign thatincludes the color and black-and-white images,the sign location, and the sign's retroreflectivityvalues. The system can also produce a graph ofthese values on a section of road as long as 80kilometers (50 miles). This graph has beenfound to be the best tool for determining whento replace signs along a given corridor.
BENEFITS
The most important benefit of METS is thatwornout signs are detected and replaced morereliably, improving public safety. Whenmandatory retroreflectivity levels for signs areestablished, all transportation agencies willneed the capability (either in house or througha contractor) to measure retroreflectivity. Ofthe three available methods—visual rating,retroreflectometer, and METS—the last or asimilar system is the most practical formeasuring the reflectivity of large numbers ofsigns in the field.
In addition, METS makes information morereadily available to analysts. As digital imagesand retroreflectivity ratings of signs arecollected over time, the performance of a givensign's sheeting can be evaluated.
Finally, METS appears to offer a costadvantage over visual rating and use ofthe retroreflectometer. In NCHRP Report346, Implementation Strategies for SignRetroreflectivity Standards, the estimated cost ofvisual rating is $3.93 per roadway mile fordaytime inspection and $5.40 per roadway milefor nighttime inspection. The estimated cost ofthe retroreflectometer approach is $14 per sign inlabor alone; traffic-control and equipmentexpenses would increase the figure. By contrast,the cost of using a METS-type system wasestimated, as part of NCHRP Project 5-10, to beonly $3.70 per sign. This figure includes laborcosts ($1.00), vehicle operating and maintenancecosts ($0.33), and capital costs, which wereamortized over 3 years ($2.37), and reflects five5-hour data collection sessions each month.Increasing the efficiency and number of thesesessions per month would decrease the figure.
For further information, contact David Long,Materials and Technology Division, MichiganDepartment of Transportation, 8885 Ricks Road,Lansing, MI 48909 (telephone 517-322-6138, fax517-322-5664, e-mail [email protected]).
EDITOR'S NOTE: Special appreciation isexpressed to Ray Derr and G. P. Jayaprahash,Transportation Research Board, for theirefforts in developing this article.
Suggestions for "Research Pays Off" topicsare welcome. Contact G. P. Jayaprakash,Transportation Research Board, 2101Constitution Avenue, N.W., Washington, D.C.20418 (telephone 202-334-2952; [email protected]).
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