National Academies Press: OpenBook

Development of Clear Recovery Area Guidelines (2024)

Chapter: Chapter 9 - Conclusions, Recommendations, and Suggested Research

« Previous: Chapter 8 - Recovery Area Guideline Development
Page 133
Suggested Citation:"Chapter 9 - Conclusions, Recommendations, and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
×
Page 133
Page 134
Suggested Citation:"Chapter 9 - Conclusions, Recommendations, and Suggested Research." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
×
Page 134

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

133   C H A P T E R   9 Summary and Conclusions Under this project, guidelines for roadside clear zones were developed in terms of key road- way and roadside design parameters. Due to the limitations inherent in using crash data for this purpose, an innovative methodology was used that combined encroachment simulations, crash data, statistical modeling, and risk analysis. A simulation matrix, consisting of over 2 million encroachments, was executed using a vehicle dynamics code enhanced by the researchers to more accurately account for various vehicle- terrain interactions that can occur during vehicle traversal of roadside terrain features. Use of computer simulation permitted a detailed analysis of vehicle trajectory and resulted in vehicle kinematics for a wide range of variables for which data are not otherwise available. The variables in the simulation matrix included vehicle type, encroachment speed and angle, vehicle orienta- tion at departure (i.e., tracking or non-tracking), driver input (e.g., steering and/or braking), horizontal curvature, vertical grade, shoulder width, foreslope ratio, foreslope width, ditch bottom width, backslope ratio, and backslope width. Simulation output included lateral distance traveled, vehicle stability outcome, trajectory data, and velocity data. Real-world crash data were used to develop probability distributions for the selected encroach- ment variables. The results were used to determine marginal probabilities for the values of the encroachment variables used in the simulation matrix that were applied as weighting factors to the simulation results. A probability matrix for vehicle type was developed using vehicle sales data. Encroachment relationships were derived from the simulation results in terms of significant roadway and roadside design variables to assist with the determination of the probability and severity of an impact given an encroachment has occurred onto the roadside. Models were devel- oped for lateral and longitudinal distance traveled by the encroaching vehicle, rollover probabil- ity, and speed and angle at a prescribed lateral and longitudinal offset. These relationships were developed for two ranges of posted speed and two facility types. The encroachment relationships were incorporated into a risk analysis tool to estimate the probability of a fatal or serious injury crash [P(K+A)] with fixed objects at the clear zone edge. An encroachment-based analysis methodology was used to estimate the conditional probability of a crash given a roadside encroachment has occurred and the probable severity of the crash. Sensitivity analyses were performed using the risk analysis tool to evaluate the sensitivity of the design variables to the estimation of risk and the relative importance of the variables to the overall determination of P(K+A). The results of the sensitivity analyses were used to determine which variables to retain or exclude from the clear zone guideline development process. Analyses were parametrically executed using the risk analysis tool to cover combinations of facility type, posted speed limit, roadway and roadside design variables, clear zone distance, and Conclusions, Recommendations, and Suggested Research

134 Development of Clear Recovery Area Guidelines obstacle spacing beyond the clear zone edge. A relative risk approach was used to define a clear zone distance for a given design configuration that has a P(K+A) risk equal to that of a guardrail. The final clear recovery area guidelines were developed in both chart and equation form in terms of the most significant design variables. The guidelines can be used to determine a recom- mended clear recovery distance for a given set of roadway and roadside characteristics and the obstacle spacing beyond the clear zone edge. For Consideration The clear zone guidelines developed under the project were formatted and presented in a manner deemed suitable for consideration in a future edition of the AASHTO Roadside Design Guide (RDG). The guidelines are presented in both equation and chart-based forms to provide options for a designer. The guidelines are based on a relative risk assessment. The risk associated with impacting fixed objects at the clear zone edge is compared to the risk associated with impacting a guardrail on the edge of the roadway. The risk-based guideline is a different approach from the previous guideline. When obstacle spacing is sufficiently large, the risk of a K+A crash is low, and the guideline may recommend a very small clear zone, even for high-speed conditions. Conversely, for very small obstacle spacing, the probability of impacting an obstacle can approach 1.0, and there may not be a practical recommended clear zone distance. In such situations, shielding the roadside with a guardrail could be considered by the designer. Previous risk-based guidelines have been developed for other design scenarios, such as deter- mining the need and appropriate test levels for median barrier (29), bridge rails (30), and shield- ing bridge piers (31). The clear zone guidelines developed under this project can be considered for inclusion in a future update of the RDG that incorporates a risk-based philosophy for road- side design. Suggested Research The clear zone guidelines developed under this project are based on the injury risk associated with striking fixed objects at the clear zone edge. Another aspect of injury risk associated with roadside encroachments is rollover crashes. The vehicle type, terrain conditions, and driver input can all influence the probability of a rollover crash. This is related to the traversability of a given terrain configuration, which is another aspect of roadside design. Future research could be performed to incorporate rollover into the clear zone guidelines. Under this project, the weight encroachment simulation data was used to develop a regression model for the probability of rollover based on lateral offset and statistically significant design variables. In addition to being influenced by variables such as shoulder width, foreslope ratio and width, ditch bottom width, and backslope ratio, it was observed that rollover increases non- linearly with increasing lateral distance. Additionally, a relationship between rollover speed and injury severity was developed under this project using the crash database in NCHRP Web-Only Document 341 (3). The rollover probability model and rollover severity relationship were incorporated into the Clear Zone Guideline Assistance Program (CZ-GAP) for use in future risk-based analyses. A comprehensive risk-based analysis that includes rollover risk could potentially result in a guideline that encompasses terrain traversability, clear zones, and guardrail need.

Next: References »
Development of Clear Recovery Area Guidelines Get This Book
×
 Development of Clear Recovery Area Guidelines
Buy Paperback | $97.00
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

The clear zone concept for roadside design emerged in the mid-1960s as a single distance for lateral clearance that reduced the likelihood of an errant vehicle striking a roadside obstacle. Subsequent recovery area guidance that evolved over the next two decades provided a variable distance expressed in terms of traffic volume, design speed, sideslope, and other roadway and roadside factors.

NCHRP Research Report 1097: Development of Clear Recovery Area Guidelines, from TRB's National Cooperative Highway Research Program, develops updated guidelines for roadside clear zones expressed in terms of key roadway and roadside design parameters. These updated guidelines can aid designers in better understanding the risk associated with roadside encroachments while recognizing and working within the associated design constraints.

READ FREE ONLINE

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!