Speed Prediction Models for Rural Multilane Highways and Urban and Suburban Arterials
III. RESEARCH PROBLEM STATEMENT
The Interactive Highway Safety Design Model (IHSDM) is a decision support tool that consists of the following modules: intersection review, traffic analysis, driver/vehicle, policy review, design consistency, and crash prediction. Through 2008, IHSDM was only able to assess planned or existing two-lane rural highways. However, in 2009, the crash prediction capabilities of the IHSDM were expanded to include rural multilane highways and urban and suburban arterials, as a faithful implementation of Part C of the first edition of the Highway Safety Manual. Methods to assess the design consistency for multi-lane rural highways and urban and suburban arterials are not available. Development of design consistency procedures for these facility types will provide a full suite of mobility and safety assessment tools for use designers throughout the project development process.
The objective of this research is to conduct basic research on speed prediction for rural multi-lane highways and urban and suburban arterials. For the purposes of this research, the project should focus on passenger car operating speeds, trucks, and recreational vehicles, and be developed in a manner that complements the existing two-lane rural highway design consistency module currently available in the IHSDM. The association between vehicle operating speeds and geometric design features on these facilities could assist in several design functions, particularly when used in concert with the latest enhancements to the IHSDM crash prediction module. Examples of this include: assessing the need for climbing lanes, justification of maximum grades, evaluating proposed capacity-expansion projects, and assessing speed-safety relationships on horizontal curves. The findings from this research could also be used as a framework to perform level of service analysis on uninterrupted flow facilities. In the Highway Capacity Manual (2000), estimating free-flow speeds are an important step in freeway and multi-lane highway operational performance.
IV. LITERATURE SEARCH SUMMARY
Previous research has focused on two-lane rural highways. Fitzpatrick et al. (2000) developed the methodology for the design consistency module of the IHSDM. This study focused on the effects of horizontal curvature (radius) and vertical curvature (curvature change rate) on 85th-percentile operating speeds. Predicted operating speeds on adjacent elements are then compared according to the consistency criteria that were established by Lamm et al. (1988).
Statistical models relating vehicle operating speeds to geometric design features is limited. Dixon et al. (1999), Figueroa and Tarko (2004), and Gong and Stamatiadis (2008) are the only studies that consider speeds on rural multi-lane highways. These studies have found an association between operating speed and the following design features: available sight distance, intersection density, available clear zone, median type, shoulder type, pavement type, approach tangent grade, horizontal curve length, and horizontal curve radius.
Statistical models to develop the association between vehicle operating speeds and geometric design features on urban and suburban arterials can be found in studies by Tarris et al. 1996, Fitzpatrick 1997, Poe and Mason 2000, Fitzpatrick 2003, Wang et al. 2006, and Nie and Hasson 2007. The following geometric design features were associated with vehicle operating speeds: available sight distance, access density, shoulder type, lane width, roadside object offset, sidewalk presence, parking presence, horizontal curve radius, deflection angle, horizontal curvature change rate, inferred design speed, and vertical grade.
V. RESEARCH OBJECTIVE
The IHSDM crash prediction module includes the Highway Safety Manual (Part C) methods to evaluate safety performance on rural multilane highways, and urban and suburban arterials. The proposed research will supplement these capabilities by developing tools to assess design consistency on these same facility types. The following is a list of tasks required to complete the proposed research:
1. Review and assess the overall methodology (including speed prediction, acceleration/deceleration, and design consistency criteria) that is currently used in IHSDM to assess design consistency for rural two-lane highways.
2. Review, and critically synthesize, the literature related to speed- geometry relationships. This review should be focused on multilane highways and urban streets.
3. Review literature related to speed modeling techniques and important speed-related performance measures. The review should focus on advancements in speed modeling methods made since original development the IHSDM design consistency module as well as other untried methods that have potential to improve speed prediction capabilities.
4. Develop a site selection and data collection plan. The plan should include locations where speed data will be collected, a list of variables that are expected to be associated with vehicle operating speeds on multilane highways and urban and suburban arterials, and an estimate of the required sample size for the proposed modeling efforts.
5. Develop a framework to evaluate design consistency on rural multilane highways and urban and suburban arterials using the speed prediction models developed by this research.
6. Prepare and submit an interim report describing the findings from Tasks 1 through 4. Meet with the project panel to discuss Phase I findings and recommendations. Revise the data collection plan and design consistency framework based on comments received during the project panel meeting.
7. Execute the data collection phase of the research as outlined in the revised plan.
8. Estimate models to determine the relationships between roadway features and operating speeds. The models should be validated and tested for geographic transferability.
9. Prepare and submit a final report describing the entire research effort. This report should include the findings from all project tasks and recommendations for implementing the speed findings into the proposed design consistency framework.
VI. ESTIMATE OF PROBLEM FUNDING AND RESEARCH PERIOD
Recommended Funding: $600,000
Research Period: 30 months
VII. URGENCY, PAYOFF POTENTIAL, AND IMPLEMENTATION
The product of this research will be an important addition to the field of geometric design. The IHSDM has been recognized to be a useful tool for analyzing both existing and proposed highway alignments. With increased focus on performance-based design, enhancing the design consistency methodology to complement the current scope of the crash prediction methodology will be highly useful for those making geometric design decisions.
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Dixon, K. K., C. H. Wu, W. Sarasua, and J. Daniels. 1999. Posted and Free-Flow Speeds for Rural Multilane Highways in Georgia. Journal of Transportation Engineering, Vol. 125, No. 6, pp. 487–494.
Figueroa, A. and A. Tarko. 2004. Reconciling Speed Limits with Design Speeds. Report No. FHWA/IN/JTRP-2004/26, Purdue University, West Lafayette, IN.
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