Seattle’s Bicycle Counters, How do they work?
I’ve always wondered how the Bike Counter at the Fremont Bridge works since I pass it often getting around my commute and when I’m headed to the Republic of Fremont (Thanks to Wikipedia for the following factoid: Fremont is a neighborhood in Seattle, Washington. Originally a separate city, it was annexed to Seattle in 1891, and is named after Fremont, Nebraska, the hometown of two of its founders Luther H. Griffith and Edward Blewett).
Here’s an abbreviated answer: There are detectors under the pavement which detect metallic objects as they cross over them counting the object.
The long answer for the really curious courtesy of the Federal Highway Administration:
Since its introduction in the early 1960s, the inductive-loop detector has become the most utilized sensor in a traffic management system. The principal components of an inductive-loop detector system include:
- One or more turns of insulated loop wire wound in a shallow slot sawed in the pavement.
- Lead-in cable from the curbside pull box to the intersection controller cabinet.
- Electronics unit housed in a nearby controller cabinet.
Figure 2-1 displays a notional diagram of an inductive-loop detector system and the vehicle and steel reinforcement elements in the roadway with which it reacts.
Figure 2-1. Inductive-loop detector system (notional).
The electronics unit transmits energy into the wire loops at frequencies between 10 kHz to 200 kHz, depending on the model. The inductive-loop system behaves as a tuned electrical circuit in which the loop wire and lead-in cable are the inductive elements. When a vehicle passes over the loop or is stopped within the loop, the vehicle induces eddy currents in the wire loops, which decrease their inductance. The decreased inductance actuates the electronics unit output relay or solid-state optically isolated output, which sends a pulse to the controller signifying the passage or presence of a vehicle.
The bicycle counters on the Fremont Bridge and Spokane Street provide valuable information about the patterns of cycling in Seattle, and an accurate count of the number of cyclists at two of the busiest cycling locations in Seattle. They allow more accurate benchmarking for goals to increase cycling as set forth in the Bicycle Master Plan. The detailed information provided by these 24/7/365 counters will enhance SDOT’s ability to measure return on investment in bicycle facilities and help identify locations for new facilities. All of this while encouraging an affordable, healthy mode of travel that reduces greenhouse gas emissions and relieves congestion. The counters upload data once a day at 5am, which is then displayed in daily, weekly, monthly, and annual running totals.
Have a great week and weekend, and enjoy a nice ride! If you pass the counter, you can say, “I know how that thing works!”