Designing a Variable
Lane Use Intersection

Fehr & Peers’ Engineering Discipline Group recently collaborated on the design of an Active Traffic Management (ATM) system for use at an intersection in Castle Rock, CO. ATM systems are common on freeways, but rarely implemented on local streets. This project was a first for Fehr & Peers, the Town, and the Colorado Department of Transportation.

The objective of the ATM is to dynamically alter the lane assignments for one approach to an intersection, in order to accommodate morning commute demand without investing a permanent, physical capacity increase.

The solution was to provide overhead electronic signs on the traffic signal mast arm that would change the lane usage during AM peak hour and allow a dual right turn onto the NB on-ramp. The concept of the sign locations and variable messages are displayed in the slider image on the left.

Our Approach:

Additional Electronic Signs

In addition to the signs at the intersection, we also designed electronic signs at two additional locations upstream of the intersection to help motorists filter into the correct lanes as they approach the intersection. To avoid confusion and conflict when the lane assignment changed, we designed an interim condition. The interim phase is intended to avoid thru/right conflicts and help to prepare drivers for the next lane assignment.


Variable message LED signs that comply with MUTCD were recommended to be attached to the traffic signal mast arms. On the more technical side, we also created a solution that would control the sign messages at various times of the day, and make sure that the signs would never be off since there are no pavement markings on the ground and no fixed signage to use.

Electrical Work

An electric control panel and cabinet were designed, containing a series of electronic relays and manual overrides that could turn the variable messages on and off with a simple input from the I-25 ramp traffic signal controller. This design also incorporated a fail-safe feature: In the event of a loss of communication with the traffic signal controller, the signs would rest in a specific lane usage state. A battery backup system was added to provide power to the signs for up to 18 hours if the power fails, and in the event the outage lasted over 18 hours, we incorporated a connection for an external generator.

Every scenario was tested to make sure that it performed the way that it was intended:


The City of Castle Rock, CO received recognition by the Colorado Chapter of the American Public Works Association (APWA) with an award for medium-sized transportation project. Congratulations to all involved on the project team on this great effort!

Does your community have a similar need for active traffic management?