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In-Service Performance Evaluations of Alternative Intersections

Description:

Many alternative intersection forms have been implemented across the US and there are many in planning stages. There is growing interest in alternative forms for safety and operations reasons, and the Federal Highway Administration (FHWA) developed four Alternative Intersection Informational Guides through the Everyday Counts program for displaced left turns (DLTs), median u-turns (MUTs), restricted-crossing u-turns (RCUTs), and diverging diamond interchanges (DDIs). The 6th Edition of the Highway Capacity Manual includes analysis procedures for these four alternative intersections. Major field evaluations of DDIs have occurred, but field evaluations of other alternative forms have been limited.

This project will assess safety and operations at DLTs, MUTs, and RCUTs, with an emphasis on safety. Design practices for each of these alternative intersection forms have generally developed organically over time. Some lead states are beginning to have established, although often informal, design practices for one or more of these forms. Many sites, though, have been built as “one-off” projects and design decisions were made with little guidance.

As the number of alternative intersection installations has increased, the variability in geometric elements across all sites has increased. This variability enables a meaningful evaluation of features unique to alternative intersections such DLT left-turn crossover intersections, DLT’s with/without right turn channelization, u-turn crossover placement and design details for MUTs and RCUTs, channelization and separation of different right-turn movements at RCUTs, pavement marking techniques to guide drivers between minor street right turn and crossover u-turn movements, to name a few. This project should inventory known DLT, MUT, and RCUT sites in the US and catalog values and variability in design features.

Driver behavior plays a major role in the operation of alternative intersections, and signing and pavement marking plays a key role in informing drivers and guiding them through a complex and sometimes counterintuitive series of events to pass through an alternative intersection. In some cases, pavement marking effectively sets the geometry of an alternative intersection and should be considered in this project’s research. For example, only pavement marking differentiates Utah’s ThrU Turn MUT designs from Michigan’s typical designs.

Safety research will be twofold. First, crash modification factors (CMFs) will be developed, and second, safety data for each site will be assessed to determine if there are optimal values or design techniques for certain geometric elements. Crash modifiation factor development should focus on DLTs and signalized RCUTs. Before/after safety performance of MUTs and unsignalized RCUTs is more established at this time. CMFs will be developed in a manner that enables incorporation into the CMF Clearinghouse and future editions of the Highway Safety Manual (HSM). Study sites should have been in operation for several years so that a sufficient amount of “after” crash data is available.

Operations research will be limited to field travel time and delay studies to assess the accuracy of HCM 6th Edition procedures for DLTs, MUTs, and RCUTs. Due to the need to visit sites in-person, this may be done at a subset of the sites used for the safety analysis. While in-person site visits will primarily focus on collecting operational data, they will also enable a review of the overall condition of the site and any maintenance issues that may be present.

Objective:

This purpose of this research effort is to quantity safety and operational performance characteristics at three alternative intersection forms – Displaced Left Turns, Median U-Turns, and Restricted Crossing U-Turns. This research will quantify the before/after safety effectiveness of converting conventional intersections to alternative intersections by developing crash modification factors. It will also qualitatively assess the safety effects of specific design elements of the three alternative intersection forms. This research will also conduct field delay and travel time studies to assess the accuracy of the HCM 6th edition procedures for these three intersection forms.

Benefits:

Improved operations and safety at alternative intersections.

Tasks:

Phase 1

  • Conduct literature search
  • Identify study/research sites and data collection methodology.
  • Identify control sites for safety analysis
  • Identify detailed geometric characteristics of study sites
  • Prepare an interim report summarizing Phase 1 activities.

Phase 2

  • Collect data at identified sites.

  • Develop crash modification factors for converting conventional intersections to each of the three alternative intersection forms.

  • Assess the safety of specific design elements of alternative intersections.
  • Assess the accuracy of the HCM models for the three alternative intersection forms.

  • Identify sections of FHWA’s DLT, MUT, and RCUT Alternative Intersection Informational Guides that should be updated based upon the findings of this project.

  • Prepare final research report.

  • Prepare NCHRP report summarizing project findings.

Relevance:

Alternative intersection forms are of great interest on a national and state level and their numbers have increased exponentially in the past decade. States are investing in these forms with a general understanding of their benefits and the best practices of their design, but would benefit from data-driven design guidelines.

Sponsoring Committee:AKD10, Performance Effects on Geometric Design
Research Period:24 - 36 months
Research Priority:Medium
RNS Developer:Pete Jenior
Source Info:The research topic was selected by the TRB Committees of Operational Effects of Geometrics (AHB65), Geometric Design (AFB10), Access Management (AHB70), Safety Data, Analysis, and Evaluation (ANB20), Pedestrian (ANF10), and Traffic Signal Systems (AHB25) at their combined meeting in July, 2014.
Date Posted:10/26/2016
Date Modified:11/14/2016
Index Terms:Crash modification functions, Crash causes, Unsignalized intersections, Geometric design, Road markings, Diamond interchanges, Interchanges, Before and after studies,
Cosponsoring Committees:ACP60, Access Management; ACS20, Safety Performance and Analysis; ACH10, Pedestrians; ACP25, Traffic Signal Systems
 
Subjects    
Highways
Design
Operations and Traffic Management
Safety and Human Factors

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