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Appendix E: Alternative Intersections and Interchanges

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Section 1: Overview

Introduction: Alternative Intersections and Interchanges

In recent years, the number of alternative intersections and interchanges has increased substantially in the United States and in Texas. As these numbers have increased, the body of data documenting the efficacy and appropriate applications of these intersections has also increased. Some of the advantages of these types of intersections/interchanges include the following:

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An improvement in traffic flow by eliminating, relocating or modifying conflict points

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Improved signal phasing and operations

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A general decrease in crashes, and particularly a reduction in more severe crashes

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A decrease in congestion and a reduction in traffic bottlenecks

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Improvement of intersection delay, Level of Service (LOS), travel time, and vehicle throughput

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The increased ability to maintain existing bridge structures

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A possible reduction in the amount of ROW required for new projects

There are many tools currently available to conduct a preliminary (Stage 1) assessment to determine if a particular alternative is viable and preferable. The FHWA Alternative Intersections/Interchanges Informational Report is a report that provides guidance on proper selection, and the FHWA Capacity Analysis for Planning of Junctions (CAP-X) is an Excel based program that can be used to evaluate selected types of innovative junction designs (eight intersections, five interchanges, three roundabouts and two mini-roundabouts) using given peak flow volumes. Additionally, the FHWA Safety Performance Intersection Control Evaluations (SPICE), an Excel based program uses safety performance functions (SPFs) in Part C of the Highway Safety Manual (HSM) to evaluate and compare the safety aspects of at-grade intersection alternatives. The SPICE tool allows the selection of default safety performance functions (SPFs) and high quality crash modification factors (CMFs) from Part D of the HSM and CMF Clearinghouse to predict crash frequency and severity of intersection control strategies.

Stage 2 analysis would include a more detailed analysis of the preferred alternatives from Stage 1. These could include use of the Highway Safety Manual (HSM) procedures and associated programs such as Interactive Highway Safety Design Model (IHSDM), and Safety Analyst for safety analysis; Highway Capacity Manual (HCM) based tools such as Synchro, Sidra, and microsimulation modeling tools such as Vissim for traffic operational analysis; and cost-benefit analysis models.

The Design and Traffic Safety Divisions will be developing suggested procedures and protocols that incorporate Intersection Control Evaluation (ICE) which is a data-driven, performance-based framework and approach to objectively screen intersection alternatives and identify an optimal geometric and control solution for an intersection.

Early in the selection process of an alternative intersection, public involvement should be conducted to educate and allow feedback from the stakeholder and local community concerning the benefits and proper function of the chosen alternative, including vehicular, pedestrian, and bicyclist accommodations.

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Pedestrian Considerations for Alternative Intersections

While pedestrian considerations for alternative intersections require a closer examination, general considerations for non-standard roadway design treatments include:

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Ensuring sight distance for pedestrians and motorists and minimizing obstructions, this is particularly important as vehicles and pedestrians are often approaching one another at non-right angles;

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Providing wayfinding for navigation;

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Safe and comfortable traversal of intersection;

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Reducing the turning geometry in slip lanes to decrease speed;

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Minimizing the number of crossing stages; and

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Reducing the number of multiple conflict-point locations for crossings.

For more information on non-standard roadway treatments, see NCHRP Report 948: Guide for Pedestrian and Bicycle Safety at (Alternative) Intersections and Interchanges.