Abstract: A portable, microprocessor-based data collection unit can be plugged directly into traffic detectors for recording event data, such as traffic volume, disconnected from the traffic detector without loss of data, and then plugged directly into a personal computer to unload the data, communicating via a serial communications cable without any additional interface device or reader. Developed with retrofitting in mind, with its own microprocessor, and battery-powered real-time clock and data storage all interconnected in one circuit, the data collection unit can use existing air switches/loop detectors and power supplies of prior traffic counters. The microprocessor, with suitable software burned into an EPROM, can operate as a microcomputer for interchangeably collecting traffic data and unloading the data to another computer via a common connector.