About 32% of registered vehicles in the U.S are equipped with automatic emergency braking or forward collision warning (FCW) systems [1]. Retrofitting vehicles with aftermarket devices can accelerate the adoption of FCW, but it is unclear if aftermarket systems perform similarly to original equipment manufacturer (OEM) systems. The performance of four low-cost, user-installable aftermarket windshield-mounted FCW systems was evaluated in various Insurance Institute for Highway Safety (IIHS) rear-end and pedestrian crash avoidance tests and compared with previously tested OEM systems. The presence and timing of FCWs were measured when vehicles approached a stationary passenger car at 20, 40, 50, 60, and 70 km/h, motorcycle and dry van trailer at 50, 60, and 70 km/h, adult pedestrian at 40 and 60 km/h, and child pedestrian crossing the road at 20 and 40 km/h. Equivalence testing was used to determine if FCW performance was similar for aftermarket and OEM systems. OEM systems provided a warning in 95% of trials, while aftermarket systems warned in 67%. Performance was similar with the passenger car, but aftermarket systems warned in significantly fewer trials with the motorcycle, dry van trailer, and child pedestrian. On average, OEM systems warned 1.1 to 2.5 seconds before impact with different targets, while the aftermarket systems warned 0.3 to 3.5 seconds before impact. The findings indicate that existing aftermarket FCW systems can address the most common rear-end crash type involving other passenger cars but not rear-end crashes involving motorcycles or heavy trucks or crashes with pedestrians, all of which are more likely to be fatal. By expanding existing testing programs to aftermarket devices, regulatory agencies and consumer information organizations can inform consumers about the technologies, accelerate adoption, and encourage aftermarket device manufacturers to improve the performance of their systems so that the devices can provide the safety benefits observed with OEM systems.