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Publication Number:  FHWA-HRT-14-020    Date:  January 2015
Publication Number: FHWA-HRT-14-020
Date: January 2015


Factors Influencing Operating Speeds and Safety on Rural and Suburban Roads


This section summarizes project findings, conclusions, and general considerations for future research.

High Friction Surface Treatment Evaluation Findings

The research team used observational beforeafter studies to evaluate HFST at four treatment sites and three control sites in West Virginia. The HFST was applied in June 2012 with the first after period data collection in August 2012 and the second after period data collection in June 2013. The research team performed operational, driver behavior, and friction evaluations. The following is a summary of the evaluation findings:

HFST Conclusions

In summary, the operational and driver behavior analyses generally found no consistent differences at the treatment sites between the before and after time periods based on the data collected in the current study. The friction analysis, however, clearly demonstrated that the friction supply increased considerably at the four horizontal curve treatment locations in West Virginia. The friction levels generally remained high for a period of 1 year after the treatment was applied. A safety analysis, currently being completed under a separate FHWA contract, will reveal further information about the safety effects of the HFST.

HFST Considerations for Future Research

The current study collected speed, driver behavior, and friction data at several HFST countermeasure and control sites in West Virginia. An observational beforeafter safety evaluation is being completed under a separate FHWA project, “Evaluations of Low-Cost Safety Improvements Pooled Fund Study (ELCSI–PFS).” Phase VI of this effort, Safety Performance Evaluation, includes the HFST safety evaluation. Future research is recommended to determine the long-term durability of the HFST by continually assessing the friction supply at the pavement–tire interface. The friction supply should be considered in future safety performance evaluations of this treatment, by considering the time-varying nature of the HFST countermeasure.

Optical Speed Bar Evaluation Findings

OSBs were implemented and evaluated at four sites in Arizona, eight sites in Alabama, and seven sites in Massachusetts. Two different designs were tested as part of this research, and a robust data collection and analysis was undertaken to individually track each vehicle thus enabling the determination of speed changes for individual vehicles. After applying the OSBs, the research team collected the first after period data in Arizona in November 2012 and the second after period data in April 2013. In Massachusetts, the research team collected after period data in December 2012. There was no second after period in Massachusetts. In Alabama, the research team collected the first after period data in October 2013 and the second after period data in December 2013. The findings of the evaluation study are as follows:

OSB Conclusions

The results of this evaluation yielded inconsistent speed reductions at all the test sites. Based on the results, the following conclusions can be drawn:

Based on the results, it can be concluded that the OSB designs used in this research were unsuccessful in reducing vehicle speeds. The effectiveness of OSBs depends on the bar spacing and marking type, and for this reason, the results of this study cannot be generalized to confirm the ineffectiveness of OSBs.

OSB Considerations for Future Research

From the results of the field trials and previous OSB evaluations, it can be concluded that OSB treatments have little or no influence driver speeds. However, if further studies are undertaken, the following modifications should be considered:

Lane-Width–Shoulder-Width Combination Evaluation Findings

The study also estimated the safety effects of lane-width–shoulder-width combinations on rural two-lane, two-way road segments. Information was collected on 886 segments (minimum length of 0.5 mi) in Illinois (541) and Minnesota (345) with varying lane-width–shoulder-width combinations. Parameters for lane-width indicators showed that, with shoulder width ignored, the expected number of total (i.e., all types and severities) crashes increases as lane width decreases, but it is difficult to distinguish the performance of an 11-ft-lane width from a 12-ft lane width. The main effect of shoulder width was a decrease in the expected number of crashes as shoulder width increased. In addition, the interaction of the lane width indicator and shoulder width showed that shoulder width has the greatest effect on safety when the lane width equals 10ft. Shoulder width also has a greater effect on safety when the lane width is 11 ft than when the lane width is 12 ft.

Lane-Width–Shoulder-Width Combination Conclusions

The results of the lane-width–shoulder-width safety evaluations show more complex (but intuitive) interactions between expected crash frequency, lane width, and shoulder width than what is currently reflected in the Highway Safety Manual CMFs for rural, two-lane roads. For any given pavement width, there are combinations of lane width and shoulder width that result in the lowest expected crash frequency (for all crash types and severities as well as for fatal-plus-injury crashes). For narrower total paved widths, the optimal lane width appears to be 12 ft. This general conclusion is consistent with conclusions from Gross et al.(48) As total paved widths become larger, there is not necessarily a safety benefit from using a wider lane, and in some cases, using a narrower lane appears to result in lower expected crash frequencies.

Lane-Width–Shoulder-Width Combination Considerations for Future Research

FHWA’s A Guide to Developing Quality Crash Modification Factors indicates that similar conclusions from different cross-sectional studies may be one way to increase confidence in CMFs derived from regression models.(81) Two separate studies, this effort together with Gross et al., have come to similar conclusions regarding lane-width–shoulder-width interactions on rural, two-lane roads using data from different States and different statistical analysis methods.(48) The findings for a safety optimal lane width for a given total paved width imply an underlying interaction between roadway cross-section dimensions, speed, and safety. No study has been able to quantify this interaction to date, although the need to consider users’ speed adaptation in safety evaluations is recognized as a high-priority research topic.(98) A before–after safety evaluation, with participating agencies willing to reallocate pavement width to different lane-width–shoulder-width combinations, may offer additional insight. Ideally, this study would also track driver adaptation over time to the lane width and shoulder width changes, specifically with respect to driver speed. This additional piece of information would enhance the interpretation of the safety findings. Increasingly robust cross-sectional studies (e.g., larger samples, additional States) are recommended in the absence of such a before–after study. Supplementing the cross-sectional safety data with a speed data-collection effort across a variety of lane-width–shoulder-width combinations would again strengthen the interpretation of the safety findings.


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