Los Angeles Spread Spectrum Radio Traffic Signal Interconnect: Practical Lessons Learned: Evaluation Report

NHTSA · 1999 · ROSA P / Booz Allen Hamilton

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Summary

This evaluation report documents the practical lessons learned from the Los Angeles Spread Spectrum Radio (SSR) Traffic Signal Interconnect Field Operational Test (FOT), conducted between 1994 and 1998. The study addressed the need for an alternative to traditional hard-wire interconnection for the Los Angeles Automated Traffic Surveillance and Control (ATSAC) system, which faced high costs and physical constraints limiting its expansion. The primary objective was to evaluate the feasibility, cost-effectiveness, and time-effectiveness of using wireless SSR communications to extend centralized traffic control coverage. The test involved deploying SSR units at 100 intersections in the Marina del Rey area, connecting them to the ATSAC system via a network architecture using Hughes Aircraft Corporation’s technology. The system utilized the unlicensed 902–928 MHz band with direct sequence spread spectrum techniques. The network employed a "multi-hopping" design where intersections were grouped into cells with a central "headend" radio and peripheral "tailend" radios. The ATSAC system required stringent performance metrics, specifically once-per-second communications processing and a 99.9% reliability target. The evaluation methodology, led by Booz Allen Hamilton and the University of Southern California, compared SSR deployment against conventional wireline methods regarding cost, time, and technical transferability. Key findings revealed significant technical challenges in meeting ATSAC’s reliability standards. Initially, cells containing up to 32 radios suffered from frequent shutdowns and reliability rates between 70% and 90%, primarily due to message corruption and latency in multi-hop transmissions. To achieve acceptable performance, cell sizes were reduced to approximately eight intersections, yielding reliability rates of 97% to 99%, though the 99.9% target was not met. The study identified that optimal RF quality required direct line-of-sight between antennas, proper grounding, and the use of higher-gain (6dB) antennas. Additionally, the unlicensed 900 MHz spectrum proved susceptible to interference from other users, such as electronic hotel keys and vehicle location systems, raising concerns about long-term stability. Deployment configurations showed that mounting radios remotely simplified installation, while remote antenna setups facilitated maintenance. The significance of this report lies in its detailed assessment of SSR viability for traffic control systems. It concludes that while SSR offers a feasible alternative to hard-wire interconnection, agencies with stringent real-time communication requirements like ATSAC face substantial technical hurdles. The findings suggest that agencies with less demanding communication protocols may achieve better results with larger cell sizes. The report provides critical guidance on network design, antenna placement, and interference management for future ITS implementations, highlighting that SSR is most effective when physical constraints make hard-wiring impossible or prohibitively expensive.

Key finding

Spread spectrum radio achieved 97% to 99% reliability at 100 intersections, falling short of the 99.9% target due to once-per-second communication constraints and environmental interference.

Methodology

field_study

Sample size: 100

Provenance

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• v2x connected vehicle