Road Wizard 12/27/2019

Topics: State Street and Collister Drive Signal Timing, Pine Street and Eagle Road


Hello Road Wizard: After construction of the fancy new intersection finished at State Street and Collister Drive, I assumed that the timing on the light would be fixed. For all the people who are coming from Collister Drive onto State Street, we have to wait minutes for the light to change, even if there are few or no cars traveling on State. This is not only inconvenient but environmentally unsound due to the unnecessary emissions from idling cars. Thank you for considering this issue, Stanford

The traffic signal at State Street and Collister Drive is coordinated with other traffic signals on State Street. For coordinated operation to be successful and provide the best traffic flow for the majority of users, State Street is favored. This means that a certain amount of time is guaranteed for these movements. This results in side streets, like Collister Drive, that may need to wait their turn.

The new intersection at Collister Drive did not change the amount of traffic on State Street, which is still the majority. The intersection is designed to provide protected opportunity for pedestrians to cross State Street and a safer opportunity to turn on and off Collister Drive.


Dear Road Wizard: It took eight light cycles to turn from Pine Street onto the northbound Eagle Road today. Thanks, ACHD you’re even worse than the Italians at maintain traffic. Anonymous

Unfortunately, my wizarding travels have not taken me to Italy, so your slight is a bit lost on me. While Pine Street is one of the busiest mid-mile roadways in the county, Eagle Road is much busier. Specifically, Eagle Road carries more than four times the traffic of Pine Street.

The eastbound to northbound left turning movement at this intersection is typically the movement with the least number of vehicles using it.  Unfortunately, to increase the green time for this movement, time would be taken from other movements in order to maintain the cycle length.  The cycle length is the amount of time during which all the movements at the intersection can be served one time.  To increase the eastbound left turning movement green time by 10 seconds, 10 seconds would have to be taken from some other movement, all of which are operating at their minimum time.  Taking even five seconds from the Eagle Rd. through movements would cause additional congestion on Eagle Road AND cause a potential safety concern, with traffic backing up onto the westbound interstate within just a few cycles.

With St. Luke’s hospital just south of this intersection, there are a lot of emergency vehicles traveling on Eagle Road through the Pine Street intersection to and from calls.  They have transponders within their vehicles that allow them to override normal signal operation in order to expedite their travel.  When this occurs, some intersection movements can have their times shortened or even skipped, depending where in the cycle the preemption occurs.  This is an important function for emergency service operation and takes priority, even at the inconvenience of others.

It’s sometimes hard to tell, but if “Anonymous” had indicated a specific day and time they experienced, ACHD might have been able to tell you if an emergency vehicle had caused a disruption to normal signal operation.

The Road Wizard would like to wish all of you a safe and happy New Year. Please send you questions to: