February 15, 2005 > Traffic signals - Why and Where
Traffic signals - Why and Where
An interview with Fremont City Engineer Norm Hughes and City Transportation Engineer Kunle Odumade, P.E.
It is often clear why traffic signals are located at certain intersections, but as pressures of increasing traffic and different commuter patterns emerge, why are some locations slated for traffic control while others are relegated to the purgatory of endless waiting? Some neighborhoods have rallied for action seeking to curb fast and dangerous traffic, trying to avoid what is considered inevitable tragedy. TCV spoke with Fremont city engineers to explain the process. These comments are related to the city of Fremont only. We will ask the traffic departments in other cities to explain their procedures in future issues.
TCV: How are intersections selected for traffic signals or controls?
Odumade: Most requests come from citizens asking for a traffic assessment.
Hughes: Citizens are our observers. When they feel there is a need, it is an indication to us to pay attention to an area. We are also out, observing, in the community.
TCV: Is there a threshold of calls to cause you to inspect an area?
Odumade: One call is enough.
Hughes: We do prioritize locations. The location should be a situation where we are aware of significant traffic volumes and warrants (justification for action) would probably be met with a study.
TCV: At a recent Fremont City Council meeting, a point system was mentioned? How does this work?
Hughes: "Warrants" used to prioritize the need for a traffic signal are: total traffic volume, interruption of continuous traffic (side street traffic including pedestrians), school crossings, accident experience, four hour volume and peak hour volume. For instance, difficulty for pedestrians crossing the street was one of the major factors for the proposed signal at Sailway Drive (at Paseo Padre Parkway).
TCV: Are all factors considered equal? How do you rank the requests?
Odumade: We monitor traffic mechanically and visually. Each of the factors is valued; volume, [traffic] interruption and pedestrian access are each rated at a maximum of 15 points. Coordination - addressing possible obstruction of traffic and pedestrian access - is rated at a maximum of 10 points. The warrant that adds the most points is accidents; that can add up to 20 points. A major part of installing a traffic signal is to improve safety.
TCV: How are accidents assessed? Is this based on the number of accidents, type of accidents, trends?
Odumade: We look at "correctible accidents." This is the rate of accidents that can be improved by installing a traffic signal. An accident caused by driving under the influence for instance, would probably not be corrected by a traffic signal and receive no points. However, a correctable severe injury accident is awarded six points.
TCV: Over what period of time are you evaluating the accident rate? Do you look at trends?
Odumade: We look at the preceding 12-month period and see how many correctible accidents have occurred. We also look at trends.
Hughes: Once an intersection is warranted by points, we use our judgment of other factors including trends to make recommendations to council.
TCV: Who makes the determination whether an accident is "correctible"?
Odumade: We look at accident reports to make a determination. Even if a particular intersection has a high level of accidents, it may not come out with a high level of points. We may do something at that intersection related to traffic control without installing a signal.
TCV: Are citizens interviewed and additional information used in making traffic control decisions?
Odumade: We go out and observe - day and night, paying attention to times of high volume and/or accidents.
Hughes: We also talk with the people who are recommending traffic control measures and others observing problems.
TCV: What happens when there is significant history of severe or fatal accidents yet the point count due to low volume does not give high priority to a signal at the intersection? For instance, a fatal correctible accident carries the weight of nine points. If two of these occurred within the last 12 months, yet volume carried little weight, the intersection might be ranked low in priority. The severity of these accidents calls out for a solution.
Odumade: In some cases, although a problem exists, another solution can be found that does not include the installation of a traffic signal. An example of this is on Mowry Boulevard between Blacow Road and Farwell Drive. Currently, a break in the median allows cross traffic to make dangerous turns. A solution, which will soon be under construction, is a partial barrier that will restrict traffic movement in certain directions.