A location's mean daily traffic (MDT) is the expected traffic volume on a "typical" day, and AASHTO's Guidelines for Traffic Monitoring Programs identifies 22 different uses for estimates of MDT. The problem of quantifying the "precision and bias" of short-count estimates of MDT is reviewed, with particular attention to the different sources of estimation errors. It is argued that early attempts to estimate the likely error of short-count estimates did not include errors from using incorrect seasonal or day-of-week adjustment factors. It is also indicated that incorrect adjustment can produce very substantial increases in estimation errors. Research on managing such adjustment errors is reviewed, leading to the conclusion that seasonal counts, rather than simple coverage counts, may be needed to produce accurate estimates of MDT when solid prior information concerning appropriate adjustment factors is lacking.