8.1.19 Collector-Distributor Roads
A collector-distributor (C-D) road is a road that parallels and connects the main travel lanes and frontage roads or entrance ramps. The purpose of a C-D road is to minimize weaving on the main travel lanes of a freeway by reducing the number of entrance and exit points on the through lanes while satisfying the demand for access to and from the freeway.
C-D roads are limited access roadways provided within a single interchange or continuously through two or more interchanges along a freeway corridor. They are similar to frontage roads except that access to abutting properties is not permitted.
The minimum lane arrangement for a C-D system is two C-D lanes and two mainlane through lanes, as shown in
; however, other combinations may be developed as capacity needs warrant.
Continuous C-D roads should be integrated to the main travel lane design to develop an overall system.
C-D roads should have shoulders equal to the width of the main travel lane shoulders.
Sometimes C-D roads are referred to as an “Intersection Bypass”. See
for an example of this application. In this case, a C-D road is similar to an exit ramp but is typically longer than a ramp. A C-D road is used to meet the following goals:
- Minimize weaving along the mainlanes and/or frontage roads;
- Optimize capacity and improve traffic operations at interchanges;
- Enhance safety by managing traffic speeds of the connected facilities;
- Minimize number of the mainlane ramps; and
- Enhance safety by providing a place for exiting traffic to wait to access cross streets, rather than backing up onto high-speed mainlanes.
Figure 8-6 Example of a Collector-Distributor System
Figure 8-7 Example of an Intersection Bypass Collector-Distributor
C-D roads are a desirable component at all cloverleaf interchanges. Where there is considerable demand for frequent ingress and egress, as in and near the business districts of large cities, a collector-distributor road, continuous through several interchanges, should be provided. See
for an example of a cloverleaf interchange C-D road.
8.1.19.1 Collector-Distributor Design Criteria
The following design criteria should be used for the design of collector-distributor roads:
- Design Speed: No more than a 10 mph reduction from mainlane design speed;
- Lane Width: freeway mainlane width ( ); and
- Capacity analysis and basic lane determination should be performed for the overall system rather than for the separate roadways.
Figure 8-8 Example of a Cloverleaf Interchange Collector-Distributor