If you’ve ever done a lot of driving on the interstate system, you know first-hand that tractor-trailer units have a much larger and different footprint than regular passenger (or light duty) vehicles. The WYDOT Connected Vehicle (CV) Pilot is focusing on improving safety and mobility by creating new ways to communicate road and travel information to commercial truck drivers and fleet managers along the 402 miles of Interstate 80 (I-80) in the State. A part of this pilot includes equipping semis with connected vehicle onboard units (OBUs) and antennas to enable communication with other equipped vehicles and the infrastructure. The tractor-trailer unit, which could consist of a semi with one or two trailers, introduces signal reception challenges not found with CV-equipped light-duty vehicles. DSRC signals require line-of-sight for reception. If the antennas are not positioned appropriately on the semi, the box trailer will likely “block” the signal broadcast. During initial installations and testing, WYDOT found that antenna performance was an issue. Tractor-trailer units were seeing a substantial DSRC shadow behind the box trailer. What this means is a light duty vehicle within 50-150 meters behind the box trailer experiences degradation of the DSRC signal from the semi. This makes forward collision warning (FCW) for vehicles (particularly light duty vehicles) coming from behind the tractor-trailer in-effective.
Wyoming CV Pilot Tractor Trailer (Source: WYDOT)
The WYDOT team worked with their vendors and other equipment suppliers to find a solution to the DSRC shadow problem. Testing1 was performed summer 2018 using multiple configurations of antennas, cables, elevations, and antenna mounting locations on the semi. Various types of antennas, cables, cable adapters, and extension bars were used as well. Testing indicated the DSRC shadow persisted. None of the test permutations resulted in a solution or improvement in DSRC signal degradation. Three possible issue areas were discussed among team members: self-interference, reflections, and space diversity for antenna placement. Further separation of antennas was posited to help with self-interference and reflection. Test procedures and test setup were documented, and an internal draft report was provided to USDOT.
WYDOT took advantage of the Saxton Transportation Operations Laboratory (STOL) Equipment Loan Program to “tryout” other connected vehicle equipment including a data packet sniffer to support testing. Having access to other connected vehicle equipment allowed WYDOT to test additional units and components in varying configurations that without the loan program would have taken much longer. Based on testing with the loaner equipment, WYDOT was able to hone-in on equipment specification and provide that information to their vendors.
In fall 2018, the WYDOT again worked with their vendors and other equipment suppliers using higher quality antennas and longer (i.e., 14.5’), low-loss cables (e.g., LMR-195). The results were very promising leading WYDOT to refine the purchase request for upgraded shark fin antennas for light duty vehicles, and upgraded dual antennas (left/right doors) with extended cable length for trucks/semis and snowplows. The LMR-195 cable has half the DB loss per foot than the LMR-100 cable. The same test procedures that had been conducted in summer 2018 were rerun. WYDOT recorded test results and provided an internal report to USDOT. Following the test procedures developed previously minimized rework and ensured test results were accurate.
The WYDOT team found that cable quality (rating) and length were significant factors contributing to the DSRC shadow issue. DSRC frequency ranges are very “loss-y” with longer antenna cable runs. Because of their size, snowplows and tractors/semis require longer cables. Higher rated cables are required for the longer cable runs. Light-duty vehicles do not require longer cables; therefore, the lower rated cables can be used for WYDOT patrol vehicles and other light-duty vehicles participating in the pilot. Quality of the antenna and components and design were found to be factors affecting performance. Antenna design including style (e.g., shark fin, omni-directional pole), and dB gain and loss associated with an OBU all contribute to the overall performance and must be factored into the design and installation. Location or placement of antenna on the tractor/semi to provide line of sight with trailing light-duty vehicles was critical. Several configurations with various antenna types were tested. These included high antenna mounts on the tractor cab, cab roof, side mirror mounts, and tractor bumper mount.
Although the tractor-trailer DSRC shadow resolution took longer than anticipated, WYDOT now has a path forward regarding equipment specification and antenna placement. WYDOT is working with vendors on antenna type, design, and cabling that are required to ensure the FCW application works whether the following vehicle is another tractor-trailer or a light duty vehicle. Details will be provided in the Comprehensive Installation Plan.
