  A SPaT (Signal Phase and Timing) message describes for each lane the current
phase at a signalized intersection together with an estimate of the residual
time of that phase. Accurate SPaT messages can be used to construct a speed
profile for a vehicle that reduces its fuel consumption as it approaches or
leaves an intersection. This paper presents SPaT estimation algorithms at an
intersection with a semi-actuated signal, using real-time signal phase
measurements. The algorithms are evaluated using high-resolution data from two
intersections in Montgomery County, MD. The algorithms can be readily
implemented at signal controllers. The study supports three findings. First,
real-time information dramatically improves the accuracy of the prediction of
the residual time compared with prediction based on historical data alone.
Second, as time increases the prediction of the residual time may increase or
decrease. Third, as drivers differently weight errors in predicting `end of
green' and `end of red', drivers on two different approaches may prefer
different estimates of the residual time of the same phase.
