Roadside Hazards

By Allan F. Davis


An important aspect to be considered in highway design litigation is the safety of the roadside environment. Since the late 1960s, highway engineers have become increasingly concerned with roadside hazards such as trees, steep embankments and shoulder drop-offs.

Statistics have shown that a significant percentage of fatal motor vehicle accidents each year involve a car that has run off the road and either struck a fixed object or rolled over. Recognizing this, various public entities and traffic engineering organizations have developed national and state design guidelines for the enhancement of the safety of the roadside. For example, California’s Department of Transportation (CalTrans) has published reports and standards since 1967, which ascribe to the concept of a clear roadside recovery area with a “forgiving quality.” This article discusses the most common roadside hazards and some of the available literature addressing such dangers.


In the late 1960s, highway researchers, including those with CalTrans, began to expand the focus of highway safety research and development to include the roadside environment. In 1967, CalTrans initiated a program known as CURE (Clean Up Roadside Environment) to establish a clear recovery area with a “forgiving quality” on freeways. That program was used for several years to fund improvements to freeway shoulders and right-of-way areas.

The current policy in California is the “Clear Recovery Zone” found in the CalTrans Traffic Manual, Section 7-02. As discussed below, the policy’s goal is to provide a hazard-free recovery zone for vehicles which run off the roadway.

A number of studies have been published on the subject of roadside hazards over the past 30 years. The 1985 CalTrans report “A Study of Fixed Objects” and the 1989 AASHTO “Roadside Design Guide” (American Assn. of State Highway and Transportation Officials) provide engineering guidelines and data for design engineers to consider in planning and improving the safety of highways.

The “Roadside Design Guide” states that from 1976 to 1986, roughly 60 percent of all fatal motor vehicle accidents involved only one vehicle. In 70 percent of those accidents, the vehicle ran off the road and either rolled over or collided with a fixed object, such as a sign post or a tree.

Among the more typical roadside hazards are: (a) vertical “drop-off” at the edge of pavement, which results in the loss of vehicle control; (b) a steep “non-recoverable” embankment or fixed object, unprotected by guardrail; or (c) fixed objects in the roadside recovery zone.

Although cross-median accidents also rank high on the highway accident severity list, and are a priority of most transportation departments, they are not regarded as roadside accidents for the purpose of this article and will not be included.

A. Pavement Edge Drop-off

Transportation officials have known for over 40 years that a drop-off at the edge of pavement can have an adverse effect on the handling and stability of a vehicle. Factors such as water run-off, wind and traffic, will cause the erosion of shoulder backing material, leaving the edge of the pavement exposed. The American Association of State Highway Officials reported in their 1954 publication “A Policy on Geometric Design of Rural Highways”: “Unstabilized shoulders frequently are hazardous because the elevation of the shoulder at the pavement edge may be several inches lower than the pavement.”

One issue which has been the centerpiece in the debate between various highway safety researchers is: At what point does a drop-off become a hazard for motorists whose vehicles leave the roadway and must then remount the pavement edge?

According to a variety of studies performed on the subject, the following factors are important in determining the influence of the drop-off on a vehicle: (a) the shape and depth of the exposed edge; (b) the nature of the adjacent unpaved surface, i.e., loose gravel can compound the effect; (c) the speed of the vehicle; and (d) whether the tires are “scrubbing” against the vertical face of pavement when attempting to remount the roadway.

Between 1975 and 1994, eleven technical papers analyzing the relationship between pavement edge drop-off and vehicle stability were published. (See References below.) The majority of the studies were initiated by state transportation agencies to generate data to establish maintenance standards, and to defend the advancing tide of lawsuits concerning edge drop-off.

Researchers, to no surprise, in all but a handful of the reports concluded that only the most severe drop-offs (i.e., equal to or greater than 4″) would adversely affect vehicle stability. John Glennon’s 1987 report, “Effects of Pavement/Shoulder Drop-Offs on Highway Safety,” was an exception. Glennon’s report listed among other things, vehicle speed, drop-off height, edge sharpness and angle of re-entry as factors which significantly affect vehicle control.

The CalTrans Maintenance Manual, from 1971 through 1975, set forth this state’s maintenance policy on the issue and revealed a more conservative viewpoint on the matter. In the section bearing the title “Dropoff” found in Chapter XI, the following language is found:

Loss of lateral support (dropoff) causes deterioration or failure at the edge of surfacing. Dropoff at the edge of surfacing can also result in loss of vehicle control. The narrower the surfaced shoulder, the more objectionable the dropoff…. [D]ropoff at the outside edge of asphaltic shoulders less than eight feet in width and between the edge of the traveled way and non-surfaced shoulders should be repaired when it exceeds 1± inches or edge failure becomes apparent. (Emphasis added.)

By March of 1976, with new data coming in from other transportation agencies around the country, CalTrans elected to re-examine the issue. In a published report entitled, “The Effect of Longitudinal Edge of Paved Surface Drop-off on Vehicle Stability,” the authors criticized their current maintenance standards as “conservative.” The Office of Transportation Laboratory in Sacramento called for an all-out review of the standards after concluding that there was no significant safety hazard in remounting edges up to 4± inches.

Tests were performed on various edge heights up to 4± inches, but the limitations in the parameters of the tests were nearly as instructive as the conclusions reached in the published report. Testing conditions did not include: high (freeway) speeds, i.e., greater than 60 mph; loose shoulder material; or an evaluation of tire “scrubbing” (a condition that occurs when the off-road tire scuffs along the pavement edge prior to remounting). These factors are important to an analysis of the severity of an edge drop-off condition. Soft or loose shoulder surface material, such as mud, gravel or sand, can allow tires to sink into the shoulder, which effectively increases the height to be negotiated. Such conditions will also adversely affect the coefficient of friction on the tires, thereby compounding the difficulty in remounting the pavement, and enhance vehicle instability. (Ivey, Don L., et al, “The Influence of Roadway Surface Discontinuities on Safety,” Transportation Research Board, National Research Council, Washington, DC, 1984.)

The CalTrans drop-off policy has been significantly modified since 1975. The Maintenance Manual now recommends that when the support (i.e., shoulder height) has diminished to approximately one-half of the pavement thickness, it should be scheduled for repair. (CalTrans Maintenance Manual, C5 [June 1998].)

According to Glennon’s 1987 report for the Transportation Research Board, scrubbing creates a unique hazard because the motorist encounters resistance forces as he steers the vehicle into the vertical face of the drop-off. The steering input increases to overcome those forces until the off-road tire is able to climb the edge. At that point, however, the driver has created such a large steer angle (toward the opposite side of the road) with large lateral acceleration and yaw velocity, that the vehicle will then suddenly veer towards the opposing lanes of traffic, forcing the driver to drastically steer back toward the opposite side. During this process, the driver typically loses control as the vehicle swerves back and forth across the road, or enter oncoming traffic, resulting in an accident.

A few state transportation departments, including Oregon, Washington and Illinois, adopted standards which required that the edge of pavement be kept flush with the unpaved shoulder surface. Los Angeles County maintenance standards require annual shoulder inspections, and mandate repairs where edge drop-offs are 2 inches or greater.

In developing evidence for use in a drop-off case, it is recommended that prompt investigation be performed to thoroughly measure and photograph the roadside conditions at the accident site, as close to the date of the accident as possible.

B. Guardrails, Embankments and Fixed Objects

In general terms, a roadside barrier or guardrail is used to protect motorists from hazards off the traveled way, such as fixed objects (i.e., trees, and utility poles) non-recoverable slopes and bodies of water. In the case of embankment slopes, highway designers and traffic engineers are left to contend with two primary factors in determining whether a barrier is needed: embankment height and slope steepness.

Chapter 7 of the CalTrans Traffic Manual provides a comparative risk warrants graph (Fig. 7-1) for embankment guardrail. Referred to as the “Equal Severity Curve,” the chart plots a curve based upon data from studies focusing on the relative severity of accidents involving vehicles which had run off the road and down an embankment, versus accidents involving vehicles which had impacted guardrail.

The “Equal Severity Curve” was developed to provide highway engineers with a basis for comparison between the relative severity of embankment accidents and guardrail accidents.

As stated in the “Roadside Design Guide”: If the consequences of a vehicle striking a fixed object hazard or running off the road are believed to be more serious than hitting a traffic barrier, then the barrier is considered warranted.

The CalTrans Equal Severity Curve (Fig. 7-1) is used by finding a point on the graph where the lines from the slope steepness (vertical line) and the embankment height (horizontal line) intersect. Based upon past accident experience involving slopes with such parameters, an accident involving a vehicle which runs off the road and down an embankment, having a height and slope which plot above the curve, will be more severe than if that vehicle were to impact a guardrail. Therefore, the barrier under such circumstances is “warranted.” An accident on a slope without a guardrail which plots below the line will be less severe than an impact with a guardrail. In that circumstance, the installation of guardrail is generally not warranted.

CalTrans warrants provide the additional caveat that guardrail should be installed “only when there is a high potential for running off the road” at the subject embankment.

Section 7-3 of the Traffic Manual provides that guardrail installation should occur at locations with a “high run-off-the-road accident history” or where there is a potential for same. Other factors, in addition to embankment height and slope, weigh-in on the determination of guardrail installation. The factors considered by CalTrans to be important in the evaluation are found in section 7-01.4 of the Traffic Manual. They include: (a) alignment of the road, i.e., run-off-the-road accidents are more common on curves than on tangents; (b) volume of traffic, i.e., the more cars on a road, the higher the probability of a run-off-the-road accident; (c) roadside recovery area, i.e., more “over the embankment” accidents will occur on roads with narrower recovery areas; (d) climatic conditions, i.e., severe weather conditions increase the potential for run-off-road accidents. (This is particularly true in mountainous areas.)

Highway engineers are quick to point out that a guardrail itself is a fixed object, and that at the right speed and sufficiently acute angle, a collision with guardrail can indeed become a catastrophic event. In 1981, nearly 2,300 fatalities occurred when vehicles left the roadway and collided with barriers. According to the National Transportation Safety Board in their 1980 report, “Safety Effectiveness Evaluation of Traffic Barrier Systems,” a roadside barrier in and of itself is a “potential hazard.” To be effective, it must be capable of safely redirecting and containing an errant vehicle without imposing “intolerable conditions” on the occupants of a vehicle striking it.

Discovery to CalTrans should include a request for all traffic collision reports for the area. From the reports come useful information as to the run-off-road accident experience and accidents which demonstrate the high probability of such occurrences. This includes spin-outs or roll-overs in the travel lanes.

Embankments adjacent to freeways and highways, in both “cut” and “fill” construction, are governed by the design criteria found in the CalTrans Highway Design Manual, section 304, “Side Slopes.” The manual generally discusses slope criteria for both inside (median) and outside slopes having their own design parameters, with the proviso that “flatter slopes are safer.” Wide unsurfaced medians (up to 20m wide) should be sloped downward to form a shallow valley, with cross slopes of 10:1 or flatter, 20:1 being preferred.

Since most freeways in California are constructed with a 10-12-foot paved right shoulder, and an inside shoulder of five feet, many run-off-road freeway accidents occur in the center median. The outcome of an accident in the center median may depend largely on the steepness of the cross slopes. The steeper the slope, the less likely a successful recovery will be.

According to the 1989 Roadside Design Guide, “critical embankment slopes” are those having a slope ratio steeper than 3:1, which will cause most vehicles to overturn. The ratio is the horizontal distance (expressed in feet) that the slope travels for each foot vertically that it drops. For example, a 3:1 slope would have that angle formed by laying one end of a yardstick flat on the ground and then raising the other end one foot off the ground.

Critical embankment slopes should be shielded by guardrail if they begin within the clear zone distance. The guide articulates that slopes from 3:1 up to 4:1 are generally regarded as “traversable,” provided that they are “smooth and free of fixed object hazards.” Most vehicles encroaching onto such slopes, however, will proceed all the way to the bottom. It is therefore important that a clear recovery or “run-out” area be provided at the bottom.

C. Clear Recovery Zone Concept

It was not until the late 1960’s that highway engineers began to adopt the concept known as the “clear zone” or “forgiving roadside.” In 1967, the AASHTO issued a publication entitled “Highway Design and Operational Practices Related to Highway Safety” (also known as the Yellow Book). The second edition in 1974 stated in regards to roadside areas: “…for adequate safety, it is desirable to provide an unencumbered roadside recovery area that is as wide as practical on a specific highway section.” This philosophy acknowledged that motorists do indeed run off the road, and that fatalities and severe injuries could be reduced by providing a “traversable recovery area.” This area should ideally be free of fixed objects such as large trees, utility poles, and signs – and have a surface which is traversable – that would allow a motorist to recover control of his vehicle in a run-off-the-road incident. Objects in the clear zone which cannot be removed or redesigned should be shielded by traffic barriers or crash cushions.

The 1989 AASHTO Roadside Design Guide provides a wealth of useful information dealing with highway roadside safety issues. In section 3.1, “The Clear Roadside Concept,” the following statistic is cited: “On high speed highways, a width of 30 feet or more from the edge of the traveled way permits about 80 percent of the vehicles leaving a roadway out of control to recover…” The guide, however, provides that the clear zone is not an exact standard; rather, it is a variable distance, depending on design speed, ADT (Average Daily Traffic volume) and embankment slope.

In Section 7-02.1 of the CalTrans Traffic Manual (11-96), the current policy on clear recovery zones on California roads is discussed as follows: An area clear of fixed objects adjacent to the roadway is desirable to provide a recovery zone for vehicles that have left the traveled way. Studies have indicated that on high-speed highways, a clear width of 9m from the edge of the traveled way permits about 80 percent of the vehicles leaving the roadway out of control to recover. Therefore, 9m should be considered the minimum clear recovery area for freeways and high-speed expressways. High-speed is defined as operating speeds greater than 70 km/h.

For conventional highways, however, that distance is not considered practical, and therefore the design standard is reduced to 6m. Designers are also advised that conditions such as traffic volume, speed, alignment, side slope, weather, adjacent development and environmental factors must be considered as well.

In the 1990 publication, “Guidelines For Application of the AASHTO Roadside Design Guide For Federal-Aid Highway Projects,” emphasis is placed on the need for a “firm and smooth roadside surface,” without which “a sideslipping car would be likely to trip and roll on the slope itself, regardless of the absence of fixed object hazards.” In a 1985 report, “A Study of Fixed Objects,” CalTrans offers accident statistics on the most common causes of run-off-road rollover accidents. A significant number of rollovers occurred from vehicles which tripped on a berm or lose material on the roadside.

In litigation involving a vehicle which has left the roadway and encountered a dangerous condition, the “clear recovery” zone concept should always be analyzed in evaluating the safety of the highway in question.


In 1963, the Legislature added Government Code 815, which in essence abolished common law forms of governmental tort liability. According to the legislative committee comments, there is no liability in the absence of a specific statute declaring there to be so, and then only under the particular conditions described in the code. Liability for dangerous roads is premised on the provisions of Government Code 835. That section requires plaintiffs to establish that an employee of the public entity created the dangerous condition; or, that the entity had actual or constructive notice of the condition a “sufficient time prior to the injury” to have allowed for remedial measures.

Several discrete theories may be alleged in the typical dangerous roadway case, including: (a) negligent design or construction; (b) negligent maintenance or repair; and (c) failure to provide warning signs. Liability against public entities is said to be cumulative, i.e., lack of evidence or an immunity problem, precluding one theory of recovery does not automatically affect the viability of another. (See Mozzetti v. City of Brisbane (1977) 67 Cal.App.3d 565; Flournoy v. State (1969) 275 Cal.App.2d 806; and Cameron v. State (1972) 7 Cal.3d 318.) Therefore, it is always advisable to plead and pursue discovery on multiple theories.

In Mozzetti, the plaintiffs advanced separate theories of recovery for flood damage to their property on the grounds of defective design of a road project and negligent maintenance of the drainage system. The court concluded that the jury could be instructed that the City may well be immune as a matter of law for “design-related damages,” but to the extent there were damages unrelated to the design (i.e., negligent construction or improper maintenance), the City might still be liable.

The Supreme Court in Cameron stated the rule as follows: By force of its very terms, the design immunity of section 830.6 is limited to a design-caused accident. [Citation.] It does not immunize from liability caused by negligence independent of design, even though the independent negligence is only a concurring, proximate cause of the accident. (7 Cal.3d at p. 328.)

In Cameron, the court held that even though the State may be potentially immune from damages based upon the negligent design of a state highway with a dangerous curve and super-elevation, it may still be liable for negligence in failing to install appropriate signs warning of the curve and safe speed for the curve. In Cameron, the court reversed a judgment of nonsuit based on evidence submitted by plaintiffs which showed that the design plans prepared in the mid-1920s contained no specification for the super-elevation of the road – which they claimed was a cause of the accident ! and thus there was no showing that the uneven super-elevation was the result of an approved plan or design.

In Flournoy, the court held that while the plaintiffs would not be permitted to recover for the negligent design and construction of a bridge – which design allowed for the formation of ice on the road surface – they could still be entitled to recovery for negligent failure to warn of the condition. The court held that the latter would be a concurrent cause of the injuries and an independent basis of recovery.

It is, therefore, essential that plaintiff’s counsel in a highway design case include multiple theories of liability in the initial claim and the complaint, and develop a discovery plan for each such theory, to prepare for the typical immunity challenges.


To fully appreciate the potential liability of a public entity in a highway defect case, it is important for counsel to have information about common roadside hazards and how they cause accidents. Many of the defects discussed in this article, such as a pavement edge drop-off, can be subtle and easily missed during the crucial post-accident investigation period. Conditions at the accident scene may change quickly in the weeks and months following the accident, resulting in the loss of vital evidence of the defect. It is therefore vital that counsel and his or her investigator know what defects to look for and how to preserve evidence for use in trial.


  1. Stannard, Baker J., “Traffic Accident Investigator’s Manual,” Fourth Revision, January 1975, Traffic Institute, Northwestern University, Evanston, Illinois.
  2. Ivey, Don L. and Griffin III, Lindsay I., “Driver Vehicle Reaction to Road Surface Discontinuities and Failures,” International FISITA Conference, Tokyo, 1975.
  3. Nordlin, E.F., et al, “The Effect of Longitudinal Edge of Paved Surface Drop-off on Vehicle Stability,” CalTrans Report CA-DOT-TL-6783-1-76-22, March 1976.
  4. Stoughton, R.L., et al, “The Effect of a Broken A.C. Pavement Drop-off Edge and Muddy Shoulder on Vehicle Stability and Controllability,” Memorandum Report, CalTrans, July 1978.
  5. Klein, Richard H., Johnson, Walter A. and Szostak, Henry T., “Influence of Roadway Disturbances on Vehicle Handling,” Contract DOT-HS-5-01223, October 1976.
  6. Ivey, Don L. and Zimmer, Richard A., “Pavement Edges and Vehicle Stability – A Basis for Maintenance Guidelines,” Research Report No. 328-1, Texas Transportation Institute, September 1982.
  7. Ivey, Don L., et al, “The Influence of Roadway Surface Discontinuities on Safety,” Transportation Research Board, National Research Council, Washington, DC, 1984.
  8. Olson, Paul L., et al, “Pavement Edge Drop,” Contract DOT-3262-5-003(B), The University of Michigan, Transportation Research Institute, July 1986.
  9. Glennon, John C., “Effects of Pavement/Shoulder Drop-offs on Highway Safety,” Transportation Research Board, National Research Council, Washington, DC, 1987.
  10. Ivey, Don L., et al, “Safety in Construction Zones Where Pavement Edges and Drop-offs Exist,” Paper No. 870523, Texas Transportation Institute, January 1988.
  11. Humphreys, Jack B. and Parham, J. Alan, “The Elimination or Mitigation of Hazards Associated with Pavement Edge Drop-offs During Roadway Resurfacing,” University of Tennessee, Transportation Center, February 1994.