Roadside workers and emergency responders, such as police and emergency medical technicians, are at significant risk of being struck by vehicular traffic while performing their duties. While recent work has examined active and passive systems to reduce pedestrian collisions, current approaches require line of sight using either laser-, infrared-, or vision-based systems. We addressed this problem by developing a Global Positioning System (GPS)-based solution that equips roadside workers and vehicles with GPS units to estimate the trajectory of oncoming traffic, and to estimate whether worker strike is imminent. The results of our study show that our approach is 91% accurate in alerting the worker and vehicle of collisions and near misses. Furthermore, accurate warnings can be provided 5 to 6 seconds before any potential collision, allowing time for mitigating solutions.