Effectiveness of a Digital Speed Displays

Effectiveness of a Digital Speed Display Trailer on a Rural Low Volume Road

Town of Clinton, Kansas
Prepared by, Brad Ayres
Douglas County Public Works | summer 2020

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Radar speed trailer provided by Kansas University Transportation Center and The Kansas Local Technical Assistance Program

Purpose

The purpose of this study was to test the effectiveness of a radar speed trailer on a low-volume rural road that is subject to higher speeds due to its location and access to seasonal water sporting activities and out-of-town unfamiliar drivers.

Introduction

The town of Clinton, Kansas is situated on the central west portion of USACE Reservoir Clinton Lake in Douglas County, Kansas. Clinton has a modest population, according to the 2000 United States Census Bureau, and covers roughly 17.3 square miles of land, with most residents living along Douglas County Route 6.

Clinton is unique due to the fact that there is essentially one route leading to and through the town. This route connects many lake activities, including a Public Beach, Boat Ramps, Campgrounds, and Fishing areas.

Background

Over the years, residents of Clinton Town have been concerned with the amount and speed of traffic, especially during the warmer months. Various attempts have been made to reduce vehicle speeds along this route. Sections of the road had different speed limits before, which have been standardized to a consistent 30 mph. Despite these changes, high-speed concerns persist, prompting residents to test the effectiveness of a radar speed trailer.

Equipment Used:

2 - JAMAR TECHNOLOGIES TRAX APOLLYON Traffic Counters/Classifiers using twin EPDM road tubes with 2-foot spacing. Layout #6, 2 Lanes 2 Directions.
1 - MPH digital speed display radar trailer with red flashing over speed display

The radar speed trailer and the traffic recorders were set up at or close to the East and West ends of town to collect data on vehicle speeds before, during, and after the study. The goal was to monitor speed changes approaching and after passing the speed trailer.

Data collection began on 6/3/2020 and concluded on 6/30/2020. The speed trailer was displayed from:

• 6/09/2020 to 6/16/2020 for Eastbound traffic - set to flash red at 31 MPH.
• 6/16/2020 to 6/23/2020 for Westbound traffic - set to flash red at 31 MPH.

The speeds shown above are 85th percentile speeds in Miles Per Hour.

The 85th percentile speed is defined as, “the speed at or below which 85 percent of all vehicles are observed to travel under free-flowing conditions past a monitored point.”

Summary of Results

As shown on the provided map, traffic recorders were strategically placed at varying distances from the speed trailer on opposite ends of town. This was done to observe speed differences before, during, and after the study for both directions. The far west recorder saw minimal change for eastbound traffic since the speed trailer wasn’t visible until drivers were past the recorder. However, a modest one MPH reduction was observed for westbound traffic after passing the trailer.

The greatest change in speeds was noted by the east traffic recorder from 6/16 to 6/23 for westbound traffic. This recorder was much closer to the trailer, resulting in noticeably different speed patterns. The summarized table below provides a breakdown of the recorded speeds and the impact of the speed trailer for each direction.

The Traffic Recorders will be named West Recorder and East Recorder:

Important data summarized above showcases a general reduction in speeds when the radar speed trailer was deployed. Notably, the EAST traffic recorder showed the most significant impact, demonstrating that proximity to the speed trailer matters.

In Conclusion

The primary goal of this study was to identify and address speeding issues through Clinton, KS. By collecting data before and after the speed trailer setup, Douglas County is evaluating whether to purchase a similar speed control device. Although the study setup wasn't perfect, the results were promising.

The EAST traffic recorder showed the most noticeable reduction in speed, likely due to its location close to the trailer. Generally, radar speed signs slow traffic by 1 mph to 11 mph. For instance, a study in Riverside, California, showed a 6.1 mph reduction at the speed sign, and a 2.9 mph reduction down the street past the trailer. Even after removing the sign, a reduction was observed. Similar results were found in Minnesota with sustained slower traffic a year after removing speed feedback signs. Conversely, Pennsylvania saw an average speed increase of 3 to 9 mph after removing radar speed signs.

So, are radar speed trailers worth the investment? Yes, if used for temporary purposes like events or road construction. However, their long-term effectiveness may diminish as drivers become accustomed to them. Evidence also suggests that some drivers might try to "get a high score," showing extremely high speeds when the trailer is deployed.

Future studies could provide additional insights from different locations.

Thank you,
Brad Ayres,
Traffic Services Superintendent, Douglas County Public Works

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