  A traffic performance measurement system, PeMS, currently functions as a
statewide repository for traffic data gathered by thousands of automatic
sensors. It has integrated data collection, processing and communications
infrastructure with data storage and analytical tools. In this paper, we
discuss statistical issues that have emerged as we attempt to process a data
stream of 2 GB per day of wildly varying quality. In particular, we focus on
detecting sensor malfunction, imputation of missing or bad data, estimation of
velocity and forecasting of travel times on freeway networks.
