This report explains how to estimate freight demand using secondary source of data such as traffic counts. Freight origin-destination (OD) matrices are one of the most important data elements a planner could have, which is why a significant amount of effort, time and money is spent on their estimation. The estimation of OD matrices can be done by: (a) direct sampling methods; and, (b) using secondary data sources such as traffic counts. The latter techniques are referred to here as origin-destination synthesis (ODS). OD data are obtained by interviewing the participants in the transportation activity and have some well-known limitations: roadside interviews tend to double count trips; on-board interviews may lead to bias in the parameters of random utility models; mail interviews are often biased because the rate of response varies across the population; and home interviews, though able to provide statistically sound estimates of OD, require a great deal of planning, time, effort and money (Ortúzar and Willumsen, 2001).