Factors Influencing Operating Speeds and Safety on Rural and Suburban Roads

CHAPTER 3. GENERAL METHODOLOGIES AND SCOPE

The research team identified several treatments for possible field evaluations during this study and based the following list of possible treatments for field evaluations on findings of the literature review and State outreach efforts:

• Converging chevron markings: Published literature shows this treatment has reduced vehicle speeds by 2 to 3 mph on rural highways; however, there have been no documented safety evaluations to date.⁽⁵³⁾ This treatment can be applied using standard pavement marking materials and seems easy to maintain.
  
  
• Narrower allocations of lane and shoulders widths on existing pavement: Design consistency literature has shown that narrowing travel lanes or shoulders is associated with reduced speed.⁽⁴⁸⁾ Recent research suggests that reallocating lane and shoulder widths on an existing pavement can possibly improve safety.⁽⁵³⁾
  
  
• Speed tables: Studies show this low-speed treatment can significantly reduce travel speeds on suburban streets, yet little is known about the safety effects of this treatment.^(53,54)
  
  
• Enhanced speed limit legend with colored surfacing: The Iowa Department of Transportation documented that this treatment can significantly reduce vehicle operating speeds, but the sample of evaluation sites used is small and no safety evaluations have been documented to date.⁽⁵³⁾ This treatment can be applied using standard pavement marking materials and is seemingly easy to maintain.
  
  
• Transverse markings or OSBs: Published literature shows that this treatment has reduced vehicle speeds by 1 to 5 mph on rural; however, there are no documented safety evaluations to date. Highways. (See references 53 and 55 through 58.) This treatment can be applied using standard pavement marking materials and seems easy to maintain.
  
  
• Red border speed limit sign: Treatment has been shown to reduce vehicle operating speeds; however, there are no documented safety evaluations to date.⁽⁵⁹⁾ The costs to manufacture, install, and maintain this treatment are low.
  
  
• HFST: A single evaluation in Florida shows that this novel treatment can reduce vehicle operating speeds along curves; however, there are no robust safety evaluations reported to date.⁽⁶⁰⁾ There appear to be several candidate evaluation sites in four States. (Note: A before-after safety evaluation of this treatment is being completed under a separate FHWA project, “Evaluations of Low-Cost Safety Improvements Pooled Fund Study” (FHWA-HRT-14-065)).
  
  
• Zigzag pavement markings: This novel treatment has shown to reduce vehicle speeds; however, there are no safety evaluations reported to date.⁽⁶¹⁾ This treatment can be applied using standard pavement marking materials and seems easy to maintain.
  
  
• Speed feedback signs: At least seven studies (not including those in school zones) show significant speed reductions result following speed feedback signs; however, the safety effects of these often short-term (less than 1 year) treatments is unknown. (See references 53 and 62 through 67.)
  
  

The FHWA technical review panel for this project organized these nine treatments into three groups: 1) treatments it has a high interest in evaluating, 2) treatments it has only a moderate interest in evaluating, and 3) treatments it has only a low interest in evaluating.

The panel’s highest interest treatments were the following, in no particular order:

• HFST.
  
  
• Narrower allocations of lane/shoulder widths on existing pavement.
  
  
• Converging chevron markings.
  
  

Its moderate interest treatments were the following, in no particular order:

• Zigzag pavement markings.
  
  
• Transverse markings/OSBs.
  
  

The panel’s low interest treatments were the following, in no particular order:

• Speed tables.
  
  
• Enhanced speed limit legend with color surfacing.
  
  
• Red border speed limit signs.
  
  
• Speed feedback signs.
  
  

The research team also assessed treatments based on State and local agency interests and their willingness to install them for this study purpose. For example, converging chevron pavement markings were not included in a treatment evaluation because of a lack of existing treatment locations on rural and suburban highways and the reluctance of agencies to install and maintain this treatment for the purposes of this study.

Based on feedback from FHWA, the current study completed the following evaluations:

• An observational before-after speed and driver behavior evaluation of the HFST on rural, two-lane horizontal curves. An observational before-after safety evaluation is being completed under a separate FHWA project, “Evaluations of Low-Cost Safety Improvements Pooled Fund Study (ELCSI–PFS)” (FHWA-HRT-14-065). Phase VI of this effort, Safety Performance Evaluation, includes the HFST safety evaluation.
  
  
• An observational before-after speed study of the OSB treatment on rural and suburban roads. In addition, the research team compiled all before-period crash data for the OSB treatment sites and comparable reference group sites into an analysis database and delivered to FHWA for future safety evaluations. At the time this final report was being prepared, less than 1 year had passed since the OSB treatments were deployed at most of the treatment sites. An adequate after-period was therefore not available for the safety evaluation. It is recommended that additional after-period crash data be collected during the years 2014 through 2016, and these data be added to the existing analysis database prior to completing the safety evaluation.
  
  
• A cross-sectional safety evaluation of the allocation of lane and shoulder width on existing rural, two-lane highways.
  
  

It should be noted that the FHWA selection of treatments to study as part of this effort is not necessarily correlated with magnitudes of expected speed reductions.

CRASH ANALYSIS FINDINGS SUMMARY

In addition to the operational and safety evaluations completed for the current study, the research team also examined State and local transportation agency police accident reports and completed

a speeding-related, clinical crash analysis. The purpose of this assessment was to determine which speeding-related crash characteristics (e.g., driver age, gender, degree of familiarity with the crash location, weather conditions, and other factors) may influence speeding-related crash occurrence. This detailed analysis was intended to provide insights into crash causation that could not otherwise be determined by analyzing only electronically coded data. To perform this evaluation, the research team reviewed police reports from the following jurisdictions:

• Bedford, Shenandoah, and Stafford counties in Virginia.
  
  
• Huntingdon County in Pennsylvania.
  
  
• Van Buren County (Lawrence Village) and Grand Traverse County in Michigan.
  
  
• Hillsborough, Brevard, and Volusia counties in Florida.
  
  

The analysis considered speeding-related crashes that occurred in rural and small urban areas (population fewer than 50,000 persons), in the years 2004 to 2008. Most local transportation agencies that provided hardcopy speeding-related crash reports did not provide data concerning the total number of crashes occurring within the jurisdiction during the analysis period. Therefore, the proportion of the speeding-related crashes in the total crashes was not known.

The research team used the coding provided on the police accident report to identify speeding- related crashes. The following three driver actions, either singly or in combination, were identified as speed-related crashes:

• Speeding.
  
  
• Driving too fast for conditions.
  
  
• Failure to maintain proper speed.
  
  

When reviewing the hardcopy police accident reports, the research team not only assessed the coding provided by the investigating officer, but also reviewed the narrative and diagram of the crash location.

The research team identified a total of 1,895 nonintersection speeding-related crashes in the nine counties (four States) listed above; the team then excluded crashes with the following characteristics:

• Crashes involving drivers under the influence of alcohol or drugs.
  
  
• Collisions with animals.
  
  
• Crashes involving motorcyclists or tractor semi-trailers.
  
  
• Crashes involving roadway surface conditions that were icy, snow-covered, or with loose gravel.
  
  

These precipitating events in these crashes might not necessarily be attributable to speeding; consequently, driver impairment, poor weather conditions, differences in vehicle size and weight, and unexpected animal crossings were considered as factors not necessarily associated with speeding. After excluding these four crash types, 586 (30.9 percent) crashes met the speeding-related crash criteria described above.

The general findings from the clinical speeding-related crash analysis indicate the following:

• Nearly 62 percent of the 586 speeding-related crashes occurred on curves. While the mileage of horizontal curve and tangent alignment data within the nine counties in the analysis were not available for the current study, literature suggests that crashes on curves are three times as likely as crashes on tangents. Assuming that the mileage of curved roadway segments is lower than tangent roadway mileage in the data analysis files used in this study, the findings appear to support past research related to speeding-related crash occurrence.
  
  
• Approximately 44 percent of all crashes on curves involved drivers younger than21 years old, or with less than 3 years of driving experience.
  
  
• A significant proportion (54 percent) of speeding-related curve crashes occurred on roadways that were likely familiar to the at-fault driver (within 10 mi of driver residence).
  
  
• Almost 59 percent of all crashes on curves occurred on dry roadways with about 45 percent of crashes occurring during the daytime.
  
  
• About 38 percent of the analyzed speeding-related crashes occurred on tangents.
  
  
• About 40 percent of all crashes involved drivers younger than 21 years or with less than 3 years of driving experience.
  
  
• Nearly 69 percent of speeding-related crashes occurred on roadways that were likely familiar to the at-fault driver (within 10 mi of driver residence).
  
  
• Almost 54 percent of all crashes occurred on dry roadways while about 73 percent occurred during daytime travel periods.
  
  

The findings from the clinical crash analysis suggest that inexperienced drivers are more likely to be involved in speeding-related crashes on curved alignments when compared with tangent alignments. In general, however, speeding-related crashes appear more likely on curved road segments when compared with tangent roadway segments. Speeding-related crashes on curves appear overrepresented at night when compared with daytime speeding-related crashes.