In this paper, we present results obtained through simulation of connected-autonomous semi-trucks that are operating in a leader-follower configuration. Autonomy is enabled in this configuration with a very lean sensor package on each truck, precision global positioning system (GPS), radar-based automated cruise control system (ACC), and dedicated short-range vehicle-to-vehicle communication system (DSRC). Our simulation includes modeling the operating environment, namely, the high-speed test track at the American Center for Mobility (ACM); the sensors, namely, GPS, ACC, and DSRC; and vehicle dynamics of semi-trucks. Simulation results in this paper are focused on measuring the safety margin of the follower semi-truck under different environmental conditions. We studied adverse weather and measured the decrease in safety margins with the increase in precipitation.