When examining crash data it is natural to first place blame on the roadway environment. For example, “if that tree hadn’t been so close to the road the driver wouldn’t have collided with it”, or “if there had been a barrier on the side of the road the vehicle wouldn’t have struck the ditch and overturned”. This natural pro- cess of analysis has caused roadway infrastructure improvements to become a highly researched area, and as a result, constitutes a major percentage of the currently im- plemented solutions to combat ROR. In 2003, the National Cooperative Highway Research Program (NCHRP) released a report entitled A Guide for Addressing Run- Off-Road Collisions. The report summarized the current documentation and knowl- edge concerning methods addressing three ROR related objectives: 1) Keep vehicles from encroaching on the roadside, 2) Minimize the likelihood of crashing or overturn- ing if the vehicle travels off the shoulder, and 3) Reduce the severity of the crash. “Tried”, “In Experimental State”, and “Proven” strategies such as rumble strips, shoulder treatments, relocation of hazardous roadside objects, and barrier design are discussed and evaluated in the context of highway infrastructure safety [17].
Many other reports and studies have been published which provide a more detailed analysis of particular infrastructure improvements. Patel et al. [18] investi- gated the benefits of shoulder rumble strips on two-lane rural highways in Minnesota. The results showed a 13 percent reduction in all single-vehicle ROR crashes. In 2009,
Table 2.1: Summary of roadway modifications categorized by objective, modified from [17]
Objective Roadway Modification
Keep the vehicle from leaving the roadway
• Shoulder rumble strips
• Skid-resistant pavement surfaces • Shoulder treatments:
- Shoulder drop-off (lip height) maintenance - Widened and/or paved shoulders
• Enhanced pavement markings, delineation, and other sig- nage etc.
Minimize the likelihood of crashing into an object or overturning if the ve- hicle leaves the roadway
• Hazardous roadside object removal • Safer roadside slope/ditch design • Enhanced markings for roadside objects
Reduce the severity of the crash
• Roadside barriers and guardrails • Improved design of roadside hardware
an NCHRP report was released which examined the effects of both shoulder and centerline rumble strips. The findings revealed single-vehicle ROR reductions of 11 percent and 15 percent for shoulder rumble strips on rural freeways and two-lane roads respectively [19].
ROR crashes can be very dangerous and difficult to control in both wet and dry roadway conditions, however, even the slightest amount of moisture on the road can reduce the tire pavement friction by 20 to 30 percent [17]. Neuman et al., in [17], explains that the New York State DOT found that by treating high crash risk sites with overlays or microsurfacing to improve the skid resistance, accident reductions up to 50 percent were obtained for wet conditions and 20 percent improvements overall. Noyce et al. [20] provides a summary of a number of pavement management studies
along with surveys from many U.S. states. Noyce’s conclusion is that while the rela- tionship between skid resistance countermeasures and crash reduction is difficult to quantify, it is nonetheless an important consideration for traffic safety, and pavement surfaces must be studied for their micro-texture and macro-texture.
Lane and shoulder widths are also a highly investigated area of research for ROR crashes. Gross et al. [21] found that wider roadway pavement widths were associated with lower crash rates as drivers had more space and time to react in emergency situations. In [22], Lord et al. notes that an additional 0.3 m (1 ft) of lane width can reduce the frequency of related crashes by as much as 12 percent with lower effects above 3.6–4.5 m (12–15 ft). They also showed that widening the paved shoulder by 0.6–2.4 m (2-8 ft) can reduce related crashes by 16 to 49 percent respectively. The composition of the shoulder can also have an added effect on ROR crashes with studies finding an increase of up to 14 percent on unpaved shoulders [23].
Irrespective of the pavement width, some vehicles will inevitably leave the paved portion of the roadway adding the potentially hazardous pavement edge drop- off (or lip-height) to the recovery equation. The vertical difference between the road surface and the unpaved shoulder can affect vehicle stability and in some cases cause drivers to scrub the inner sidewall of the tire creating a dangerous situation. The Federal Highway Administration (FHWA) recommends a 30◦–35◦ asphalt fillet to im- prove tire transition and avoid scrubbing citing a 5.7 percent non-significant reduction in total crashes on rural two-lane highways [24]. Hallmark et al. [25] notes an in- creased crash risk for lip-heights exceeding 5 cm (2 in), which characterized 12–18.6 percent of roadway samples taken in Iowa and Missouri. Hallmark supports the 5 cm maintenance threshold adopted by many states in the U.S.
a significant impact on ROR crash severity. Neuman et al. [17] explains that the most common harmful events in fatal single-vehicle crashes are overturn (42%), followed by impacts with a tree (26%), a utility pole (7%), and a ditch or embankment (5%). Neuman explains that the wider the clear zone the safer it will be. While researching road safety in New South Wales, Levett indicates, in [26], that 75 to 85 percent of the safety benefit associated with clear zones is obtained within the first six meters. In addition to removing hazardous objects, the slope of the clear zone can influence the chance of successful recovery. Zegeer et al. [27] indicates that significant flattening such as a 1:3 slope to 1:7 is related to lower single-vehicle ROR crashes.
Another solution for making shoulders safer are guardrails and barriers. Al- though this method adds a roadside object, it can help reduce the severity of a ROR crash by lowering the speed of out-of-control vehicles and/or redirecting them away from hazardous shoulder conditions. In [26] it is recommended that when a run-off area is available all roadside hazards should be removed and any slopes leveled out. However, when there is no run-off area available, appropriate safety barriers should be installed. In [28], Paulsen et al. found that energy absorbing guardrail terminals were capable of reducing the speed of small vehicles up to 75 percent for head-on col- lisions and 50 percent when struck at an angle of 15 degrees. Paulsen notes that the strategic placement of guardrails away from roadside slopes and embankments can greatly reduce the rollover risk. Additional studies on various guardrail and barrier designs and effectiveness can be found in [29, 30, 31, 32].
Finally, a number of enhanced delineation, signage, and pavement/roadside object marking techniques have also been implemented to grasp driver’s attention, provide a better visual of the roadway, and ultimately help prevent vehicles from leaving the roadway. Neuman et al. summarizes these countermeasures in [17], stating successes in ROR crash reduction of up to 15 percent in some cases.