Videos from cameras onboard a moving vehicle are increasingly available to collision reconstructionists. The goal of this study was to evaluate the accuracy of speeds, decelerations, and brake onset times calculated from onboard dash cameras (“dashcams”) using a match-moving technique. We equipped a single test vehicle with 5 commercially available dashcams, a 5th wheel, and a brake pedal switch to synchronize the cameras and 5th wheel. The 5th wheel data served as the reference for the vehicle kinematics. We conducted 9 tests involving a constant-speed approach (mean ± standard deviation = 57.6 ± 2.0 km/h) followed by hard braking (0.989 g ± 0.021 g). For each camera and brake test, we extracted the video and calculated the camera’s position in each frame using SynthEyes, a 3D motion tracking and video analysis program. Scale and location for the analyses were based on a 3D laser scan of the test site. From each camera’s position data, we calculated its speed before braking and its average deceleration during braking, and we then compared these values to the reference speed and average deceleration, respectively, to estimate a bias and uncertainty for each camera and kinematic parameter. Across the 5 cameras tested here, speed estimates varied from an underestimate of 0.45 ± 0.05 km/h (bias ± uncertainty) to an overestimate of 0.80 ± 0.16 km/h, and average deceleration estimates varied from an underestimate of 0.012 ± 0.013 g to an overestimate of 0.049 ± 0.012 g. The video-based brake onset times lagged the actual brake onset times for four of the five cameras, varying from 14 ± 41 ms before brake onset to 94 ± 29 ms after brake onset. These data show that, for cameras of the quality tested here, dashcam video can be used to estimate vehicle speed, average deceleration, and the time of brake application with relatively small biases and uncertainties that vary between cameras.