Traffic Intersections, Signal Lengths and Collisions in Seattle
Traffic Intersections
The City of Seattle has over 1,100 signalized traffic intersections. Typically found in downtown and along major arterial streets, signalized intersections are designed to safely direct traffic at congested intersections where stop signs would be inadequate for managing traffic flows. Interestingly, Seattle covers approximately 84 square miles, yet around half of all traffic lights are found in the 2 square mile greater downtown area (solely 2.4% of the City).
Yellow Light Lengths
Yellow lights at traffic intersections warn the driver that the signal is about to turn red. Drivers are allowed to enter intersection when the light is yellow, but are in violation of running a red light once it turns red. A well engineered yellow light length accounts for the posted speed of the roadway and the approach grade (e.g. the higher the speed limit and the steeper the grade, the longer the vehicle stopping time thus the longer the yellow light length). Using methodology per the Institute of Transportation Engineers, we calculated recommended yellow light lengths for typical posted speeds and grades found in Seattle.
Traffic Intersections
The City of Seattle has over 1,100 signalized traffic intersections. Typically found in downtown and along major arterial streets, signalized intersections are designed to safely direct traffic at congested intersections where stop signs would be inadequate for managing traffic flows. Interestingly, Seattle covers approximately 84 square miles, yet around half of all traffic lights are found in the 2 square mile greater downtown area (solely 2.4% of the City).
Yellow Light Lengths
Yellow lights at traffic intersections warn the driver that the signal is about to turn red. Drivers are allowed to enter intersection when the light is yellow, but are in violation of running a red light once it turns red. A well engineered yellow light length accounts for the posted speed of the roadway and the approach grade (e.g. the higher the speed limit and the steeper the grade, the longer the vehicle stopping time thus the longer the yellow light length). Using methodology per the Institute of Transportation Engineers, we calculated recommended yellow light lengths for typical posted speeds and grades found in Seattle.
Red Light Cameras
Of the 1,100 signalized intersections in the City, a small percentage have red light traffic safety cameras. These cameras automatically detect vehicles that run red lights, and issue violations. Currently, the City operates 31 red light traffic safety cameras at 23 different arterials intersections.
The initial launch of this program in 2006 was used to evaluate how cameras might reduce the frequency of red light running and associated accidents. The programs was expanded in 2008, 2009, and 2013. Four factors were weighed in choosing locations for the new cameras: right-angle crashes, serious pedestrian injuries, the frequency of red light running based on video observation, and geographic distribution. Through June 2015, the City's red light cameras have captured 291,813 violations.
Yellow light lengths that are too short in duration for the roadway conditions unfairly lead to red light violations.
Intersection Collision Data
Before we delve into our analysis, one other element to bring to the mix is intersection collision data. We pulled five years of recorded collision data from the City:
Of the 1,100 signalized intersections in the City, a small percentage have red light traffic safety cameras. These cameras automatically detect vehicles that run red lights, and issue violations. Currently, the City operates 31 red light traffic safety cameras at 23 different arterials intersections.
The initial launch of this program in 2006 was used to evaluate how cameras might reduce the frequency of red light running and associated accidents. The programs was expanded in 2008, 2009, and 2013. Four factors were weighed in choosing locations for the new cameras: right-angle crashes, serious pedestrian injuries, the frequency of red light running based on video observation, and geographic distribution. Through June 2015, the City's red light cameras have captured 291,813 violations.
Yellow light lengths that are too short in duration for the roadway conditions unfairly lead to red light violations.
Intersection Collision Data
Before we delve into our analysis, one other element to bring to the mix is intersection collision data. We pulled five years of recorded collision data from the City: