South Carolina is just completing a significant roadwork program where bonds have been leveraged so that 27 years of maintenance and construction activities could be completed in just seven years. During this period, the number of South Carolina interstate work-zone-related crashes and fatalities has been rising, with the state accounting for nearly 10% of the nation’s work-zone fatalities. The number of crashes in work zones in South Carolina nearly tripled in five years, increasing from 677 in 1998 to 2,601 in 2003. In all of these years, a leading cause of vehicle crashes is driving too fast for conditions. Because of the increasing number of work-zone crashes and fatalities in South Carolina, research into innovative ways to improve driver attention and reduce vehicle speeds in work zones has become a priority for the South Carolina Department of Transportation (SCDOT).
For the past several decades, transportation agencies have taken different approaches to reduce speeds in work zones, including using traffic-control devices, design alterations and police enforcement. Although it is generally agreed that law enforcement has the most significant effect in lowering speeds, this measure is often unavailable due to work force limitations and cost.
In order to address South Carolina’s need for safer work zones, researchers at Clemson University experimented with various traffic-control devices to reduce vehicle speeds on interstates and primary and secondary highways. One of the devices studied was drone radar. Drone radar simulates the presence of law enforcement by transmitting the same radar frequency, thus activating the radar detector in use in either passenger vehicles or tractor-trailer trucks. Radar detector use in passenger cars is legal except in the District of Columbia, state of Virginia and U.S. military installations, according to the FCC. According to Speedlabs.com, approximately 12% of tractor-trailers still use the device even though radar detectors were banned in all commercial vehicles by all states in a U.S. DOT directive in February 1995.
Releasing a Cobra
Drone radar has been studied extensively over the past 20 years, but only one published study has been conducted in the past five. Further, only a few studies have actually looked at radar-detector usage, which is a critical factor in the effectiveness of drone radar.
The primary purpose of the drone radar evaluation in this research was to investigate the effectiveness of the device in multiple types of work zones under both day and night conditions to determine the potential successful scenarios for deployment. Research was conducted to identify various types of drone radar and radar detectors to be purchased for the study. The second step in the evaluation process included an investigation of the operating limitations of drone radar to enable comparative test environments in the experimental tests. The third step included the development of a methodology for testing several qualitative and quantitative measures with the aid of a radar detector, a radar detector detector (RDD), a laser and a radar speed gun, CB radio and a communication device to relay messages between data collectors.
The researchers chose the Cobra XT 1000 Safety Alert Traffic Warning Systems drone radar, which emits K and KA-band. Tests were conducted to determine the optimal mounting specifications of the drones in the field. The tests included placement of the drones on different objects and in different terrains while using multiple brands of radar detectors to test the signal strength of the drones. The objective was to discern which radar detectors worked the best with the drone radar and to find placement locations (direction, mount and terrain) where the drone signal transmitted best.
Preliminary tests using the Cobra XT drone radar showed that it has limitations. It works best in flat areas where hills and other objects do not obstruct the signals. It works best if elevated to avoid low-lying obstructions. Orientation of the drone radar also affects its performance.
Based on these results, a simple mounting structure was developed to provide optimal signal detection length, as well as allow for quick installation of the drone. The complete apparatus costs approximately $250. The drone is attached to the top of a steel post that is mounted with a rechargeable battery pack. The post and battery assembly is painted green to blend with surrounding vegetation in an attempt to make it difficult for drivers to identify the drones.
One serious limitation found in the preliminary tests was that not all radar detectors that are specified to detect K and KA-band detect the drone. This was found to be most apparent in low-cost radar detectors. This was especially problematic for this study because there was no way to know whether or not a radar detector that is in use in a vehicle is actually sensing a drone.
A little slow
Evaluation of the drone radar began in August 2005 after contacting SCDOT construction and maintenance engineers to determine the locations of work-zone projects throughout South Carolina. One criterion for selecting work-zone sites required a high level of service within the site to allow vehicles to travel at free-flow conditions, thereby verifying the drone radar independence from other factors that may influence speeds. Sites were divided amongst interstate freeways and primary and secondary highways.
Speed data was collected for two conditions at each site: with the drone radar activated and without the drone activated. Other methods used to verify the effect of drone radar included monitoring radar detector usage, CB radio users and the volume of both passenger cars and tractor-trailers in the traffic stream to stratify the results by vehicle type.
Radar detectors were identified with an RDD from Hill County Research that uses a VG-4 frequency. The RDD was positioned near the beginning of the work zone, perpendicular to traffic flow. Using visual inspection, the researcher then separated the vehicles into passenger car or tractor-trailer. In addition, individual speeds of those equipped with radar detectors were recorded to see if they decreased their speeds upon encountering the drone radar. After the RDD identified a vehicle containing a radar detector, the speed was recorded and a description of the vehicle was radioed to a data collector downstream.
CB radio transmissions provided the researchers with any communications that may confound the results of the study. Specifically, the researchers listened for any messages about police enforcement, the identification of the drone or a data collector being spotted. All vehicular volume was recorded for both passenger cars and tractor-trailers using a handheld clicker counter.
The data collected for the drone radar were divided into three groups for analysis: passenger cars, tractor-trailers and radar-detector users. Statistical tests were conducted to analyze the significance of the effect of drone radar on work-zone speeds for the different vehicle types.
In general, results from this study show a 2-mph decrease in mean speeds of all highway vehicles and a 6-mph decrease in those equipped with radar detectors, as indicated in the table below. This table combines all the sites’ mean speed reductions for both secondary and interstate roadways for the entire traffic stream. As expected, tractor-trailers have a higher reduction in mean speeds on interstates because secondary roads have lower design speeds than interstate facilities.
The ranges in percentage of radar detector use for the various types of roadways and vehicles included in this research are, on average, slightly higher than the findings of a study done by Georgia Tech in 2000. It should be noted, however, that the RDD used in the Georgia Tech study has been found to be less reliable than the RDD used in this study.
The low radar detector use in South Carolina verifies the results shown in mean speed reductions for the various types of work zones. Overall, the drone radar is significantly effective only when looking at the mean speeds of those equipped with radar detectors, with speed reductions ranging from 4.6 to 7.9 mph.
A comparison between mean speeds of the entire traffic stream and those equipped with radar detectors demonstrates a major difference between the two groups with the drone radar off. Radar detector users are traveling much faster. However, when the drone is activated, the opposite trend occurs.
As part of this research, a survey of state departments of transportation was conducted focusing on speed-reduction strategies. The survey indicated that there are a wide variety of strategies being employed across the U.S. with varying degrees of success. Four out of 20 respondents indicated limited or test use of drone-radar applications in work zones to reduce speeds. The drone-radar devices were primarily installed on DOT construction vehicles and contractor vehicles. All of the four respondents stated that the drone radar had fair effectiveness for controlling speed in work zones. However, none of the respondents had used RDD to determine the number of radar detectors in the traffic stream. Two evaluations were cited with mixed results.
Works for now
This research determined the optimal deployment conditions for drone radar and evaluated its effectiveness as a speed control device in five South Carolina work zones. Overall, the drone radar caused minor reductions in mean speeds, 85th percentile speeds, and percentage of vehicles exceeding the speed limit. However, this technology caused significant decreases in the mean speed of vehicles equipped with radar detectors, which indicates the effectiveness of drone radar is dependent on the number of radar detectors in the traffic stream.
The study also showed that radar-detector users travel, on average, faster than non-radar-detector users. Thus, the drone slowed down many of the chronic speeders. One of the findings when developing specifications for the drone radar was the discovery of a radar detector that failed to detect the signal of the drone. This inexpensive radar-detector model purchased at a department store may be one of many models that fail to detect drone radar.
The drone radar studied in this research satisfied the objective provided by SCDOT for an affordable and easy-to-implement technology to reduce speeds in work zones. The less-than-$250 cost of drone radar is much more affordable than other traffic-control devices such as radar-equipped changeable message signs, which range from $10,000 to $20,000. One side benefit of the drone radar is the possibility of alerting fatigued drivers as they drive through work zones The drone radar should not be limited to work-zone conditions, because the low cost of this technology potentially allows their use for non-work-zone applications. The long-term effects of drone radar as a speed reduction measure were not evaluated in this study. Previous research suggests that its effectiveness decreases with time. However, this conclusion could not be verified in this research. Radar-detector users’ mean speeds are significantly higher than those without these devices. The drone radar also decreases speeds of vehicle platoons if a radar detector user is at the front.
The following recommendations can be made to improve the effectiveness of drone radar as a speed reduction measure in work zones:
- A single drone should not be used for work zones longer than a mile because drivers may speed up after the detection no longer exists;
- The drone radar should be elevated to avoid lower obstructions and faced in the proper direction to optimize transmission distance;
- Multiple drones should be placed in work zones consisting of rolling terrain to maintain a longer detection period;
- The drone radar should be placed in advance of the work-zone activity to slow vehicles prior to entering a heavy work area; and
- The drone radar should be turned off during non-operation hours of the work zone to maintain effectiveness for those using the road daily.
