The good news is that the Airflow Deflector Inc. AngelWing the IIHS tested worked well. As you can see in the video above, it effectively stops the 35-mile per hour car from slipping under the trailer. In the end, the impact looks like a more traditional front-end impact test. You can see that all of the safety devices with which the car was equipped did their jobs and protected the passenger.
The bad news is that air deflectors designed only for improving aerodynamics offer essentially no protection. The IIHS also tested one of these fiberglass air deflectors under the same conditions as the purpose-built guard. When the of car hit the deflector, it folded up against the underside of the trailer. The car went underneath, and the edge of the trailer collided with the windshield, and eventually the dummy's head. The IIHS says this collision would have been fatal.
The IIHS says this test shows that side guards can be effective at preventing deaths from this type of accident. The organization reports that 301 people died in 2015 in similar crashes. It recommends that these guards be made required equipment nationwide to prevent future fatalities.