15.6 General Design Considerations
15.6.1 General Design Considerations
15.6.1 Interchange Spacing
Interchange spacing can have a major influence on freeway operations. Appropriate spacing of interchanges is impacted by several items including interchange type, lane configuration, weaving volumes, signing, signal progression, and lengths of speed-change lanes.
The recommended minimum interchange spacing in rural areas is 2 miles. In urban areas, the desirable interchange spacing is 1-mile.
Where closer spacing is desired or required, the use of braided ramps or collector-distributor roads is recommended. Braided ramps are ramps that cross over each other and are vertically separated. These ramps separate incoming and exiting traffic by having one ramp pass over the other, thereby eliminating traffic weaving, improving safety, and easing congestion.
Traffic operational analysis will be required where lower values of interchange spacing are used.
Interchange spacing is measured between the centerline of cross streets as shown in
.
Figure 15-21: Interchange Spacing Between Cross Streets
15.6.2 Lane Balance
See
for a detailed discussion of lane balance on Freeways and their application to grade separations and interchanges.
15.6.3 Auxiliary Lanes
A continuous auxiliary lane should be constructed between the entrance and exit terminals of interchanges where the distance between the successive painted noses of an entrance-exit (EN-EX) ramp combination is less than 2,000-ft on a freeway or 1,500-ft on a collector-distributor or frontage road.
An auxiliary lane may be introduced as a single exclusive lane or in conjunction with a 2-lane entrance. Refer to
for more information concerning the termination of auxiliary lanes. The design of auxiliary lanes should be based on the traffic volumes for the exiting, entering and through movements through the procedures can be referred to in the
which is applicable to all freeway contexts.
See
for additional information regarding auxiliary lanes.
15.6.4 Weaving Sections
Weaving sections occur when entrance ramps are closely followed by exit ramps and vehicle paths cross each other. Such areas present special design problems due to the concentrated lane changing maneuvers of merging and diverging traffic. The development of the design involves the following factors:
- Desired mobility standard;
- Length;
- Number of lanes;
- Traffic volumes;
- Weaving and non-weaving vehicles; and
- Average speed.
For design guidance refer to Design Controls and Criteria, Chapter 2 of
and from Chapter 24 of the
See
for additional information on weaving lengths and ramp spacing.
15.6.5 Collector-Distributor Roads
See
for a detailed discussion of collector-distributor roads and their application to grade separations and interchanges.
Refer to Chapter 10 of
for discussion on other General Design Consideration topics.