14.1 Introduction

An alternative intersection or interchange is one where traffic movement is re-routed from a conventional intersection form to remove major conflict points. To help reduce intersection related crashes, and as traffic demand increases, the use of alternative intersections, such as roundabouts, diverging diamond interchanges, restricted crossing U-turns, and continuous flow intersections, is becoming more prevalent within the United States. The target speed should be lowered as vehicles approach and go through roundabouts and some other alternative intersection types to achieve proper functioning and the resulting safety benefits.
Some of the advantages of these types of intersections or interchanges include the following:
  • A general decrease in conflict points and crashes, and particularly a reduction in higher-severity crashes;
  • An improvement in traffic flow by eliminating, relocating, or modifying conflict points;
  • Improved signal phasing and operations;
  • A decrease in congestion and a reduction in traffic bottlenecks;
  • Improvement of intersection delay, Level of Service (LOS), travel time, and vehicle throughput;
  • Improved ability to maintain existing bridge structures; and
  • A possible reduction in the amount of ROW required for new projects.
This chapter discusses the following sections and subsections:
14.1 Introduction
14.2 Pedestrian Considerations for Alternative Intersections
14.3 Intersection Analysis/Intersection Control Evaluation
14.4 Roundabouts
  • 14.4.1 Overview;
  • 14.4.2 Goals of a Roundabout Design;
  • 14.4.3 Design Process and Design Context;
  • 14.4.4 Design Parameters;
  • 14.4.5 Performance Checks;
  • 14.4.6 Special Considerations; and
  • 14.4.7 Preliminary and Final Design.
14.5 Continuous Green T-Intersection (CGT)
  • 14.5.1 Overview;
  • 14.5.2 Design Considerations;
  • 14.5.3 Sight Distance;
  • 14.5.4 Access Management; and
  • 14.5.5 Pedestrian and Bicyclist Considerations.
14.6 Restricted Crossing U-Turn Intersection (RCUT)
  • 14.6.1 Overview;
  • 14.6.2 Design Considerations;
  • 14.6.3 Intersection Angle;
  • 14.6.4 Pedestrian and Bicyclist Considerations;
  • 14.6.5 Access Management;
  • 14.6.6 Safety;
  • 14.6.7 Operations; and
  • 14.6.8 Traffic Control Devices.
14.7 Displaced Left-Turn Intersection (DLT)
  • 14.7.1 Overview;
  • 14.7.2 Design Considerations;
  • 14.7.3 Pedestrian and Bicyclist Considerations;
  • 14.7.4 Access Management;
  • 14.7.5 Safety;
  • 14.7.6 Operations; and
  • 14.7.7 Traffic Control Devices.
14.8 Median U-Turn Intersection (MUT)
  • 14.8.1 Overview;
  • 14.8.2 Design Considerations;
  • 14.8.3 Pedestrian and Bicyclist Considerations;
  • 14.8.4 Access Management;
  • 14.8.5 Safety;
  • 14.8.6 Operations; and
  • 14.8.7 Traffic Control Devices.
14.9 Quadrant Roadway Intersections
14.10 Diverging Diamond Interchange (DDI)
  • 14.10.1 Overview;
  • 14.10.2 Design Considerations;
  • 14.10.3 Sight Distance;
  • 14.10.4 Horizontal Alignment Alternatives;
  • 14.10.5 Auxiliary Lanes;
  • 14.10.6 Ramp Terminals;
  • 14.10.7 Pedestrian and Bicyclist Considerations;
  • 14.10.8 Access Management;
  • 14.10.9 Safety;
  • 14.10.10 Operations;
  • 14.10.11 Traffic Control Devices; and
  • 14.10.12 Illumination.
14.11 Contraflow Left Interchange (CLI)
14.12 Single-Point Urban Interchange
14.13 Roundabout Interchange
14.14 Partial Cloverleaf Interchange
Refer to the
TSAP
for additional discussion on the different types of alternative intersections and alternative interchanges including:
Alternative Intersections
  • Roundabout;
  • Continuous green-T (CGT);
  • Restricted Crossing U-turn (RCUT);
  • Displaced Left-turn Intersection (DLT);
  • Median U-turn (MUT); and
  • Quadrant Roadway
Alternative Interchanges
  • Diverging Diamond Interchange (DDI);
  • Contraflow Left Interchange;
  • Single-Point Urban Interchange (SPUI);
  • Roundabout; and
  • Partial Cloverleaf.
The public should be involved in the selection process when determining the appropriate alternative interchange or intersection. Refer to the National guidance documents referenced in each respective section of this chapter for additional guidance on public involvement best practices.