Roadside Clear Zones on Roadway Curves ? Can We See Clearly Now?

John C. Glennon, D. Engr., P.E.
January 2007 (copyright)

          In the mid-1960’s, the roadway engineering community began to understand and focus on safety issues related to the roadside. What became increasing clear was that a very large percentage of all collisions involved errant vehicles leaving the traveled way. What emerged was the concept of a forgiving roadside. Efforts were exerted to flatten side slopes, to remove large and/or rigid fixed objects, to install breakaway devices, and to install traffic barriers. Increasingly these efforts were guided by objective concepts and by innovative hardware. Two of the prevailing concepts were clear zones and recoverable foreslopes.
This paper explores these concepts. It is not intended to develop new clear-zone or recoverable slope guidelines but, rather, to suggest that current guidelines need to be seriously examined, particularly from the standpoint of safe foreslopes on the outside of roadway curves.
Is the Definition of Clear Zone Really Clear ?
          Any discussion of highway roadside clear zones needs to start with a general definition of what is a clear zone. The AASHTO Roadside Design Guide1 defines clear zone as "the total roadside border area, starting at the edge of the traveled way available for safe use by an errant vehicle.” But AASHTO is not clear about what defines "safe". Implied is that a clear zone should be clear of rigid objects, – such as trees, poles, and bridge structures – which cause severe occupant injuries. Also implied is that foreslopes should be relatively flat, such as to not cause severe occupant injuries.
What is a Safe Roadside Foreslope ?
            For 40 years up until 1984, AASHTO2,3,4 recommended a maximum foreslope of 4:1, based apparently on intuitive engineering judgment. In that year, they5 began saying that, although 4:1 slopes or flatter are generally desirable, 3:1 slopes are recommended only where site conditions do not permit use of flatter slopes. This 3:1 recommendation seems to have derived from research by Ross, et. al.6 that compared the relative impact severities of foreslopes and guardrails, wherein the impact severity of 3:1 slopes was found equal to the impact severity of guardrail. With this logic, AASHTO adopted a backdoor definition of the maximum level of collision severity as being those consequences associated with guardrail impacts.
            AASHTO defines foreslopes of 3:1 or flatter as traversable and foreslopes of 4:1 or flatter as recoverable. Research by Weaver Marquis and Olson7, however, shows that the relative occupant forces manifest when a vehicle hits the bottom of a 4:1 slope are tolerable, whereas the occupant forces for a 3:1 slope are not tolerable. Therefore, if a force criterion is used, 4:1 foreslopes, rather than 3:1 foreslopes, should be used to define a reasonable threshold of safety. And, carrying this reasoning one step further, a clear zone might be defined as an unencumbered roadside border area that has a 4:1 or flatter foreslope and is free of rigid injury-producing fixed objects (to include such things as curbs, dikes, pavement edge drop offs, and other vertical discontinuities).
            With the above discussion, the force criterion yields one way to judge foreslope safety. But, a remaining question is “what defines recoverable?” Employing a safe recovery criterion where the errant driver is attempting to recover to the traveled way, what slopes present a threshold for loss of control for both tangent and curved roadways? After a discussion of roadway curve accident characteristics, the following section will explore what is a safe roadside foreslope on the outside of a roadway curve.
Roadway Curve Accident Characteristics
            Roadway curves are a necessary and important element of nearly all highways and streets. Their form has evolved from what appeared reasonable to the builder’s eye to the more modern geometrically designed form of a circular curve. Despite a reasonably well conceived design procedure, which considers a tolerable level of lateral acceleration on the driver, roadway curves continually show a tendency to be high-accident locations. Several studies over the years have indicated both that roadway curves exhibit higher accident rates than straight roadway sections, and that accident rate increases as curve radius decreases. But, curve radius may be just one element that is interdependent with other elements that together contribute to accident rate. The roadway curve is one of the most complex features of streets and highways.
Most research studies dealing with roadway curves have come to the same basic conclusion, namely that curves are hazardous. Such conclusions, however, are meaningless by themselves. The ultimate question is, under what conditions are roadway curves particularly hazardous? Findings from some studies indicate answers to these questions. Kihlberg and Tharp8 discovered that curves in combination with intersections resulted in greater accident rates. Billion and Stohner9 found that overall poor alignment resulted in higher accident rates. Babkov and Coburn10 reported accident rates for various curve radii. They found those roadway curves with radii less than 2800 feet are 20 to 50 percent more hazardous than straight roadway sections. Jorgensen 11 reported an approximate 15 percent higher accident rate for radii less than 1900 feet. Taylor and Foody12, studying curve delineation, found that the length of curve as well as its radius has an influence on accident rates.
            A 1983, four-state study, by Glennon, Neuman, and Leisch13 , is the most comprehensive analysis of roadway curve safety ever undertaken. This study compared the accident experience on 3304 rural two-lane curve segments to 253 rural two-lane straight segments. Each segment was one kilometer (0.6 mile) long and was selected to minimize variance associated with intersections, bridges, nearby urban development, and nearby curvature. Some of the more significant conclusions of this work were as follows:
l. The average accident rate for roadway curves is about three times the average accident rate for straight roadway segments.
2. The average single-vehicle ran-off-road accident rate for roadway curves is about four times the average single-vehicle ran-off-road accident rate for straight roadway segments
3. Roadway curves have a higher proportion of fatal and injury accidents than do straight segments.
4. Roadside character (roadside slope, clear-zone width, coverage of fixed objects) appears to be the most dominant contributor to the probability that a roadway curve has a high reported accident rate.
5. Although roadside character is the dominant accident factor on roadway curves, most curves with high-accident rates usually have one or more other factors that contribute to the total hazard (e.g., sharper curvature, longer curve lengths, narrower shoulders, and lower pavement skid resistance).
6. Existing roadway curves, which are significantly substandard for the prevailing roadway speeds, may pose considerable safety problems. Drivers do not totally decrease their open roadway speeds to match the safe speed of a substandard roadway curve.
7. Roadside slope traversals on roadway curves appear more severe than on straight segments. Severity is defined by the vehicle path angle to the slope, which is a function of roadway curvature. More severe traversals lead both to generally higher vertical decelerations and higher potential for rollover. These results suggest that, for comparable safety levels, roadside slopes on roadway curves need to be flatter than those on straight segments.
 Although these conclusions may vary by radius and length of curve, they do show that roadway curves are considerably more hazardous than straight segments, that single-vehicle run-off-road accidents are a prevalent aspect of curves, and that roadside accidents tend to be more severe than multi-vehicle accidents.
What’s a Safe Roadside Foreslope on the Outside of a Roadway Curve ?
The following discussion assumes the selection of a 4:1 roadside foreslope to define the threshold of safety for tangent roadways and then analyzes what roadside foreslopes will have similar thresholds of safety for the outside of roadway curves. Inanalyzing the recoverability of an errant vehicle on a curved roadside, it is informative to look at a critical circular path that is just below the threshold of loss of control. The first step in this analysis, therefore, should be to select controlling lateral acceleration levels. Here, a study by Rice and Dell14 suggests that drivers attempting to control their vehicles, particularly at higher speeds, cannot tolerate lateral acceleration levels above about 0.3 g. Also, in order to be consistent with prevailing AASHTO1 roadside safety policy, the critical path design speed should be selected as 60 mph.
For a vehicle steering in a circular path against a roadside foreslope, Newton’s second law yields
            S2 = 85,935 (f-e) / Dv
Where             S   = speed, in mph
                        Dv = degree of vehicle path, degrees per 100 feet.
                        f   = lateral acceleration, g’s
                        e   = foreslope, in ft/ft
Using the selected values of 60 mph for design speed and 0.3 g’s for design lateral acceleration, the following equation can be written for the maximum foreslope:
            e = 0.0419 Dv – 0.30ccccccc    
            The next step of this analysis requires a model of errant vehicle recovery. Here, we will apply an exploratory rational approach for the solution of critical foreslopes. Assume that the vehicle leaves the traveled way on a tangent to the roadway curve, proceeds for 100 feet along that path, and then begins a path that will steer the vehicle back to the traveled way within an additional 200 feet. In the first 100 feet, the vehicle path will deviate from the roadway curve direction bythe degree of the roadway curve, Dc To regain the roadway in 200 feet, will require a path that is 1.5 Dc. With these assumptions, the maximum foreslope is computed as follows:
      e = 0.06285 Dc – 0.3                 
Using this equation, the maximum foreslopes are shown in Table 1 for each degree of roadway curve.
Degree of
Radius of
Curve (ft)
Degree of
Maximum Foreslope
1. A 3:1 roadside foreslope is undesirable on roadways with 60 mph speeds or higher.
2. From the standpoint of roadside safety, roadway curves sharper than about 3.5 degrees are undesirable on roadways with 60-mph speeds.
3. As roadway curve degree increases (or as radius decreases) flatter roadside foreslopes are needed to provide the same level of safety provided by any given foreslope criterion used for straight roadways.
4. For sharper roadway curves, of perhaps 4.0 degrees of curvature or greater, any roadside foreslope is unsafe.
5. Because of both more frequent roadside encroachments and more severe features, roadsides on roadway curves are extraordinarily hazardous.
6. The best alternative to roadside foreslopes on sharper roadway curves is roadside backslopes.
1. American Association of State Highway and Transportation Officials, Roadside Design Guide, 1983, 2001, 2004.
2. American Association of State Highway Officials, A Policy on Highway Types, 1940.
3. American Association of State Highway Officials, A Policy on Geometric Design of Rural Highways, 1954
4. American Association of State Highway Officials, A Policy on Geometric Design of Rural Highways, 1965.
5. American Association of State Highway and Transportation Officials, A Policy on Geometric Design of Highways and Streets, 1984.
6. Ross, H>.E., Warrants for Guardrails on Embankments, Highway Research Board, Record 460, 1973
7. Weaver, Graeme D., Marquis, Eugene L., and Olson, Robert M., Selection of Safe Roadside Cross Sections, Transportation Research Board, NCHRP Report 158, 1975.
8. Kihlberg, J.K., and Tharp, K.J., Accident Rates as Related to Design Elements of Rural Highways,Transportation Research Board, NCHRP Report 47, 1968.
9. Billion, C.E., and Stohner, W.R., A Detailed Study of Accidents as Related to Highway Shoulders, Highway Research Board Proceedings, 1957.
10. Babkov, V.F.,and Coburn W., Road Design and Traffic Safety, Traffic Engineering and Control, September 1968.
11. Jorgensen, Roy and Associates,Cost-Safety Evaluation of Highway Rehabilitation Policies, 1972
12. Taylor, W.C., and Foody, T.J., Curve Delineation and Accidents, Ohio Department of Highways, 1968.
13. Glennon, J.C., Neuman, T.R. and Leisch, J.E., Safety and Operational Considerations for Design of Rural Highway Curves, Federal Highway Administration, 1983.
14. Rice, R. S., and Dell, Amico F., An Experimental Study of Automobile Driver Characteristics and Capabilities, Calspan Report No. ZS-5208-k-1, 1974.
About the Author

Dr. John C. Glennon is a traffic engineer with over 45 years experience. He has over 120 publications. He is the author of the book "Roadway Safety and Tort Liability" and is frequently called to testify both about roadway defects and as a crash reconstructionist.


john c. glennon, Book , books,pavement edge drop expert, pavement edge drop off expert, guardrail expert, work zone safety expert, construction zone safety expert, roadway hydroplning expert, traffic engineering expert, traffic sign expert, traffic signal expert, pavement marking expert, highway safety expert
Crash : John C. Glennon, Chartered Contact Us Links Home