Road Management & Engineering Journal
Copyright © 1999 by TranSafety, Inc.
Fax: (360) 335-6402
We offer our readers a forum on the safety issue of temporary pavement markings applied in a work zone. The increasing number of rehabilitation and repaving projects underway on our nation's highways makes these markings crucial.
We are beginning to see an increase in the number of suits brought against agencies and contractors for inadequate pavement markings in work zones.
Unfortunately, it does not appear that an acceptable set of striping guidelines has yet been found. In light of this, the number of suits alleging inadequate guidance through work zones may well increase in the next few years. Improvements in safety and accident reduction can come with better use of common sense in some pavement marking situations. Others require more thorough research and implementation where understanding is lacking.
The first piece presented here is a condensation of a Texas Transportation Institute (TTI) study. This was presented at the 1988 Annual Meeting of the Transportation Research Board (TRB). We have quoted verbatim from the study's abstract, introduction and findings.
Following that, we print a critique of the TTI study by Anita Ward of Potters Industries, Hasbrouck Heights, NJ. Ms. Ward is with the Highway Products Division of Potters Industries, a firm dedicated to road safety research and manufacture of pavement marking material.
The TRB regularly invites discussion of research, and Ms. Ward's critique was prepared in response to that open invitation. Ms. Ward provided a copy of her critique to TranSafety, Inc. Her response, as well as an opportunity for rebuttal by TTI, will be included in TRB's reprint of the TTI study in a Transportation Research Record.
We believe a forum for critical discussion of research findings is of great importance, and not enough opportunities have been provided for such reviews in the field of highway research. Such a process can only improve the overall quality of the research. We leave our readers to judge the merits of the TTI research reviewed here, as well as the critique by Ms. Ward.
FIELD STUDIES OF TEMPORARY PAVEMENT MARKINGS FOR WORK ZONES
ON TWO-LANE RURAL HIGHWAYS
by Conrad L. Dudek, R. Dale Huchingson, F. Tom Creasey and Olga Pendleton-
-Texas Transportation Institute, 1988
In response to the Federal Highway Administration's proposed rule making concerning the requirement that all states use 4-ft pavement markings on 40-ft centers as temporary markings in highway workzones, the National Cooperative Highway Research Program awarded a research contract to the Texas Transportation Institute to conduct field studies to compare the safety and operational effectiveness of 1-ft, 2-ft and 4-ft temporary broken line pavement markings in work zones. The following scope and test conditions were specified by NCHRP:
Introduction: Statement of the Problem
The cost of temporary traffic control is very significant on many construction, maintenance and utility projects. With the prospects of continued inflation, limited resources, and high interest rates, it is imperative that all aspects of temporary traffic control be evaluated for economy in application and benefits to the public.
FHWA has issued guidelines and proposed changes to the Manual on Uniform Traffic Control Devices (MUTCD) regarding temporary markings for construction and maintenance areas. Less than the full standard marking pattern (10-ft stripe on 40- ft centers) would be permitted for broken lines, but the proposed changes would require a minimum pattern of 4-ft stripes on 40-ft centers (36-ft gaps), which is more than double what many states now specify. There was concern that if adopted as the national standard, 4-ft markings will significantly increase project costs.
The NCHRP awarded a contract to the Texas Transportation Institute (TTI) to determine whether the proposed 4-ft markings on 40-ft centers would actually result in significant safety and operational improvements in comparison to current practice.
Findings and Suggested Research
Based on driver performance and driver subjective evaluations, the 1-ft and 2-ft on 40-ft centers striping patterns performed as well as the 4-ft pattern for centerline striping at night at seven pavement overlay projects on rural two-lane, two-way highways with 2.0 degree horizontal curvatures, level-to-rolling terrain, and average speeds between 50 and 62 mph. Studies were conducted in four states.
The findings should not be generalized to situations not tested. Nighttime viewing in an ambient background of near darkness will enhance the contrast of the bright reflective yellow stripes. Moreover, the horizontal curves were 2.0 degrees, with the exception of one curve which was 3.0 degrees. It is possible that the performance of the three tested striping patterns may not be equal on horizontal curves with greater curvature, or at urban or suburban construction zones where the ambient lighting is different than the conditions studied. Also, the three striping patterns tested may not result in the same driver performance on mountainous highways and other types of highways with lower operating speeds.
The study did not attempt to optimize spacing or brightness to determine the most cost- effective striping pattern. Although the three striping patterns tested provided adequate delineation on rural two-lane, two-way highways, they may not necessarily represent the optimum patterns from a cost-effectiveness standpoint. It is possible that patterns with larger spacings may also provide adequate path delineation on rural two-lane, two-way highways.
The limitations of this research relative to scope of the field studies were discussed in the previous section. The discussion suggests that the following additional research be conducted:
AN INDUSTRY RESPONSE TO TTI'S "TEMPORARY PAVEMENT MARKINGS"
by Anita Ward
The flawed methodology and data analysis of this field study led to conclusions that are not only highly questionable, but potentially dangerous. With the emphasis on restoring and rehabilitating existing facilities, both the driving public and construction crews are facing ever-increasing exposure to work zone activities. Potential for driver error is greatest in such areas of change in the normal environment, and safe negotiation requires clear path guidance. The "adequate" treatment reported in this study of nearly ideal conditions should not be considered as any indication of adequate performance of the pavement marking pattern in real world conditions.
Unrealistic Test Conditions
Within the scope and test conditions specified by the National Cooperative Highway Research Program (NCHRP), the Texas Transportation Institute (TTI) chose a set of near perfect conditions. On the pavement overlay projects on rural two-lane, two-way highways:
These last two points have a major impact in skewing the research results. Drivers actually see pavement markings as a function of their contrast with the road surface. New pavement overlays such as those in the test sites are generally very black, providing excellent contrast to the yellow marking tape. The retroreflective properties of the newly applied marking tape itself provide an extremely bright optical target. On a clear dry night (each one in this study), it is far easier for a driver to see such highly visible pavement markings than in most driving situations. The only visual distractions appear to have been limited traffic and the data collection system of "computers housed in vehicles parked off the roadway as far from the operating lanes as possible." (Note: There is no discussion of potential change in driver behavior as a result of the parked vehicles.)
One condition placed by NCHRP is likely to have further skewed TTI's results: the absence of edgelines. Experience and an ample body of evidence indicates improved driver performance in the presence of edgelines. With no indication of lane boundary and limited visual information at the edge of pavement in this study, drivers' focus on the centerline was even more acute.
With a wide expanse of blacktop in a nearly dark environment and a brilliant ribbon of yellow to follow as in this study, drivers should perform relatively consistently. It is not surprising that TTI's summary of traced vehicles indicates no differences in driver performance between the 1', 2' and 4' striping patterns with the measurement criteria of speed, lateral distance encroachment, erratic maneuver and speed profile data. Similarly, it is reasonable to expect that given the strong visual target of highly retroreflective, new pavement marking tape contrasted against a newly surfaced road in a background of near darkness, some individuals in the subject evaluations could not differentiate between the 1' and 2' stripes, or even perhaps between 2' and 4' stripes. Each is perceived as very bright spots in a black environment. Such spots may also appear elongated by the relatively high speeds. The TTI observation that drivers could differentiate the 39' spacing separating the 1' spots of bright light but could not discriminate between the 38' and 36' spacing separating the 2' and 4' bright spots supports this.
Perhaps the most surprising result of the field study was that even with these ideal conditions, each method of subject evaluation reported the poorest results with the 1' stripe and a preference for the 4' stripe. Yet far more important is the reported finding that none of the treatments was judged as extremely effective. The treatments were only rated 2 on a scale of 0-4. This is consistent with a prior TTI research study which reported that drivers rated 8' stripes with 32' gaps as the best striping treatment.
Given all this, it is imperative that the data in this field study are not interpreted as representative of a pavement marking pattern. It can at best be indicative of a newly placed, highly retroreflective pavement marking tape on a resurfaced road where there is little or no visual "clutter."
Need for Enhanced Safety in Work Zones
The "typical" construction zone does not meet test conditions selected for TTI's study, and work zone safety is becoming a more critical issue. Analysis of U.S. traffic accidents reveals that work zone fatalities have increased from 490 in 1982 to 680 in 1985. [AASHTO's Committee on Highway Traffic Safety] conducted a survey of work zone accidents on the Interstate and Primary system in 1985. Their summary reported:
Work zones are particularly hazardous because they present drivers with change in the normal driving environment. Such change places greater demand on drivers, which can lead to confusion and accidents. Up to 90% of all the information used by drivers to guide and control their vehicles is obtained visually, and the pavement itself is a primary information source for drivers. In fact, if a driver is presented with conflicting information, he will generally choose to follow the pavement.
Pavement markings through work zones should provide a clear path for drivers' guidance. Such markings must be effective where needed most: at night, under adverse weather conditions, and when drivers may have other visual limitations such as advancing age, fatigue or alcohol consumption. The need for strong delineation patterns in work zones is gaining widespread acceptance, and our court system is providing impetus for action. Both Louisiana and New Mexico were held liable for wrongful deaths where striping was not in place to warn and guide motorists through work zones. North Carolina has taken the lead in providing increased information through construction work zones by using 8" markings, twice the standard marking line width.
Driver and Construction Worker Expectations: Safety and Mobility
Safe driving requires both appropriate visual information and drivers that are able to receive and interpret that information. However, studies indicate that in most construction zone accidents, the driver receives neither visual stimulation nor sufficient warning. The fact that drivers often fail to meet the challenges of work zones is documented by studies indicating accident rate increases in work zones during construction as compared to before construction. Drivers can not effectively control their vehicles without sufficient visual information, and even this current TTI study indicates that a pavement marking pattern of short stripes with long gap ratios does not provide an effective level of communication. There is a significant body of evidence to indicate that driver performance is enhanced through stronger pavement marking patterns.
The negative consequences of this [TTI] report could be far-reaching. Even though it states "that the findings should not be generalized to situations not tested," response to this presentation at the annual Transportation Research Board meeting indicates that this is precisely what will happen. The potential detrimental impact to safety and mobility is heightened by TTI's own conclusions: With evidence only of treatment (with highly retroreflective marking tape under ideal conditions), TTI has claimed not only that the striping patterns of 1', 2' and 4' on 40' centers are adequate, but that even larger spacings may help to optimize cost effectiveness.
As indicated in the "Statement of the Problem," TTI uses a very narrow interpretation of the word cost. Cost is not just money spent. More importantly, cost is measured in value received. If a driver cannot safely position his vehicle through a work zone to prevent harm to those individuals or objects in the area and to protect himself and his passengers, a responsible jurisdiction should not open that area to traffic. Sound business considerations and concern for the public welfare dictate comprehensive resource management. Inadequate pavement marking patterns, especially in work zones where drivers need enhanced visual communication, are a prime example of false economy.
Copyright © 1999 by TranSafety, Inc.