By Gerald Smith
Anyone living within earshot of Highway 20 through the valley has to wonder sometime about the nearly constant traffic flow on busy weekends and holidays. In his classic columns (Off the Wall), Bob Spiwak chronicled his traffic counts at West Boesel between Winthrop and Mazama. In his last Labor day count of 2017, more than 400 vehicles were westbound during the holiday morning hour.
However, one characteristic of traffic flow that Spiwak never documented was the speed of the vehicles. In this report, speed data are included in the holiday traffic tabulation.
Determining the speed of an object in motion requires both measurement of a distance through which the object moves and the time period involved. In the case of vehicles on a highway, the speeds are relatively fast and hence for any reasonable distance, the times involved are quite short. A manually operated stopwatch would not be sufficiently accurate for careful measurement. Being a biomechanics researcher, the tools for motion analysis are familiar to me — these tools provide the technical capability for determining vehicle speeds from an inconspicuous vantage point well off the roadway.
At the speed limit of 60 miles per hour (mph), a vehicle is traveling 88 feet per second. The distance of 88 feet provided a convenient starting point for the measurements as it would take one second for the vehicle to move through that distance. To obtain accurate and precise time measurements, video recording can be used if a sufficiently high frame rate is available for a given time period. In this case, after comparing 60-, 120- and 240-frame per second (fps) recordings and finding very little advantage to higher frame rates, the analysis was carried out with the 60 fps recording using a Panasonic Lumix DC-LX100 II camera with video resolution of 1920 x 1080 progressive scan. Displayed in QuickTime 7 Pro software, individual frame numbers are displayed and can then be used to count the number of video frames between two events. In this case, the two events were the entry into and exit from a designated space along the roadway.
To mark the space along the roadway, two white PVC poles were placed well off the pavement but in a line from the camera to where the line intersected the midline of the roadway. The angles were sighted so that the midline intersections were 88 feet apart. From trigonometry, the distances along the edge lines of the road were 74 and 102 feet. However, vehicles were generally centered in each lane and not on the midline or the edge line. It was assumed that when centered in the lane, a vehicle’s near side would be 3 feet from the edge line or the midline. These distances were similarly calculated and used in the computations of speed.
In this manner, the two factors making up a speed calculation were determined. The video frame number data for vehicle entry and exit were recorded into a spreadsheet along with the appropriate distances for each lane. Speed was calculated as distance divided by time and converted from feet per second to miles per hour for reporting. Vehicle data were recorded between 9 and 10 a.m. on Labor Day, Sept. 2, 2019, along Highway 20 near milepost 179 between the Mazama intersection and Freestone Inn.
Results and discussion
Labor Day was sunny with no weather complications for travel or for the video analysis of this report. During the one-hour data collection, 325 vehicles traveled through the collection site. Of these, most were westbound (277 or 85%). Only 48 (15%) were eastbound on Highway 20. By vehicle type, 58% were cars, 33% were pickups, 5% were motorcycles along with a few RVs and trucks.
As a test of the accuracy of the video analysis methods, a vehicle was driven through the recording site in both westbound and eastbound directions at an approximate speed of 50 mph as indicated on the vehicle speedometer. From the video data, the calculated speeds were 51.0 mph (westbound) and 50.2 mph (eastbound). Differences, in this case, were likely due to either systematic error within the vehicle speedometer system, driver difficulty with holding speed constant, positioning within each lane, and small errors in the video measurement process. These illustrate the likely magnitudes of uncertainty inherent in the analysis process.
Average speeds combining all vehicles in each direction were similar: 62.4 ± 5.0 (westbound) and 63.3 ± 5.9 mph (eastbound). However, these mean and standard deviation values don’t fully convey a complete picture of observed vehicle speeds. A histogram shows the number of vehicles within small ranges of speeds, expressed as a percent of the total 325 vehicles. A majority (72% of all vehicles) were traveling faster than 60 mph and many (1 in 4) were exceeding 66 mph which is 10% faster than the speed limit. About 7% of vehicles exceeded 70 mph. Maximum speed observed was 79.8 mph (westbound) and 76.0 mph (eastbound).
The number of vehicles being operated at relatively high speeds on a two-lane rural highway is concerning. From simple physics, the kinetic energy of a vehicle at 70 mph is about 36% greater than one at 60 mph. This directly affects braking distance to a stop and the potential for catastrophic accidents.
Holiday and weekend traffic on Highway 20 through the Methow valley is considerable — at peak flow hours, it is likely much higher than observed here between 9 and 10 a.m. With a substantial fraction of the vehicles traveling at speeds well over the speed limit and with numerous serious crashes occurring each summer between the Methow and the west side, what can be done to slow the flow and calm the overly energetic drivers to safer speeds?
Gerald Smith lives in Mazama.