Periodically Perform a Sensor Calibration in An Open Space to Avoid Distorted Compass Reading from Smart Phones.
Accessible Pedestrian System Deployment in Minnesota Based on a Smartphone App Reveals System Implementation Lessons.
Made Public Date


United States

Deploy and Test a Smartphone-Based Accessible Traffic Information System for the Visually Impaired


This project leveraged the smartphone-based accessible pedestrian system (APS) developed by Minnesota DOT and extended previous work by installing the system at four signalized and two un-signalized intersections located in downtown Stillwater, Minnesota. Researchers interfaced with the traffic controllers to broadcast traffic signal phasing and timing (SPaT) information through a secured and private wireless network for visually impaired users. Bluetooth Low Energy (BLE) technology was used to identify a person’s GPS coordinates when traveling and ensured that the person was in the right location. The project’s main goal was to test the smartphone-based accessible system and evaluate the efficiency of the system for visually impaired individuals, through signalized and un-signalized intersections.

Lessons Learned

The following lessons learned are summarized:

  • Periodically perform a sensor calibration in an open space because the orientation information from the digital compass on the smartphone could be distorted when the phone is near a large ferrous metal object in the environment. 
  • Avoid placing the phone too close to the signal pole while requesting intersection geometry and signal timing information.
  • Consider alternatives to avoid data transmission latency for receiving real-time SPaT information using smartphones. Lightweight client server publish/subscribe messaging transport protocol messaging protocol was suggested instead of hypertext transfer protocol (HTTP)-based interface to address latency and timely transmit real-time signal phasing and timing data.
System Engineering Elements