PurposeTo investigate the impact of smartphone viewing distance and angle on reaction times. DesignA prospective, self-controlled, single-center study. MethodsParticipants engaged in a driving simulation facing a large screen with a simulated brake pedal. They were tasked to stop the simulation once recognizing the deceleration of upcoming traffic. Tests were conducted without distraction and with a standardized distraction simulating texting while driving (TWD). Smartphone positions varied at distances of 30 cm and 60 cm, and at angles parallel to and 30 degrees below the road plane. Reaction times were measured from the onset of simulated closure to detection. Stopping distances were extrapolated using data from the National Highway Traffic Safety Administration. ResultsNinety-four participants were included with a mean age of 24 ± 2.7 years. The control reaction time was 11.5 ± 4.1 seconds. Reaction times significantly decreased with smartphone placement at a closer distance of 30 cm parallel (17.0 ± 3.3 seconds) vs 60 cm parallel (15.4 ± 3.8 seconds), P<.001. A 30-degree downward placement at 30 cm (18.6 ± 4.0 seconds) and 60 cm (17.9 ± 3.6 seconds), further decreased reaction time compared to parallel phone positioning, P < .001. Extrapolating to stopping distances based on real-world data, smartphone distractions placed at 30 cm 30 degrees below the dashboard had the greatest effect, resulting in a 3 times increase of stopping distance compared to the control, 1201 vs 394 ft respectively, P<0.001 ConclusionTWD significantly delays reaction time in young participants. Both the distance and viewing angle of a smartphone significantly influences reaction times during driving simulations. The greatest delays are observed when the smartphone is positioned closer to the user and at a 30-degree angle which we hypothesize is due to vision blur from increased accommodation, loss of stereopsis, and fixation with the peripheral retina.