In the absence of pedestrian crossing lights, finding a safe moment to cross the road is often hazardous and challenging, especially for people with visual impairments. We present a reliable low-cost solution, an Android device attached to a traffic sign or lighting pole near the crossing, indicating whether it is safe to cross the road. The indication can be by sound, display, vibration, and various communication modalities provided by the Android device. The integral system camera is aimed at approaching traffic. Optical flow is computed from the incoming video stream, and projected onto an influx map, automatically acquired during a brief training period. The crossing safety is determined based on a 1-dimensional temporal signal derived from the projection. We implemented the complete system on a Samsung Galaxy K-Zoom Android smartphone, and obtained real-time operation. The system achieves promising experimental results, providing pedestrians with sufficiently early warning of approaching vehicles. The system can serve as a stand-alone safety device, that can be installed where pedestrian crossing lights are ruled out. Requiring no dedicated infrastructure, it can be powered by a solar panel and remotely maintained via the cellular network.