As the New Year begins, Neighborhood Traffic Safety Services will be conducting several new traffic studies that measure vehicle speed and volume in Bellevue’s neighborhoods. These studies are an integral component in how we analyze a street to determine traffic conditions.
With holiday travel behind us and school back in session, now is a great time to study our neighborhood streets for representative speeds/volumes. Do we send a staff member in the field to guess how fast cars are going? Of course not (our perception of vehicle speeds is usually very different from reality), we hire a consultant or use in-house staff to set a series of pneumatic tubes across the roadway that accurately and reliably capture both speed of individual vehicles and also aggregates how many vehicles pass over the tubes into daily counts.
Perhaps you’ve seen these black rubber tubes on neighborhood streets or on arterials. Seemingly low-tech, these tubes collect vehicle data over time (we usually conduct 7-day counts to ensure we understand the difference in travel behavior during peak weekdays, usually Tuesday through Thursday, while also including the weekend; typically, there is a steep drop-off in vehicle volumes between weekday and the weekend). Here is how speed studies are generally conducted:
- A street is selected for the speed/volume study by city staff or a resident. The street is analyzed for the best place to place the tubes; generally, mid-block locations away from intersections or other obvious impediments that aren’t representative of the streetscape are preferred.
- The tubes are nailed into the roadway by a trained technician. Precise, parallel placement of the tubes is critical to ensure accuracy.
- Vehicles travel over the tubes for the duration of the study. As vehicles travel over the tubes, blasts of air are sent to the receiver box and the data is stored in the box until the technician picks up the equipment. With a known distance between the parallel tubes, speeds are calculated by measuring the time difference between the front axle hitting the first and second tubes.
- After the field component is completed, the technician retrieves the equipment and compiles the data to send to staff.
- Staff receive the data, interpret the results, and communicate a summary to residents (often in the form of a Traffic Action Plan).
Though traffic studies are important in understanding conditions on the roadway, it’s by no means the only step in determining whether there is a traffic safety issue. The studies are a first step that complement other analysis that create a holistic view of what is going on in the roadway. However, that’s not to say we haven’t collected a lot of data in the city over the years…