As agencies place more emphasis on monitoring and managing the roadway network, the concept of traffic monitoring systems based on wireless location technology (WLT) is generating increasing interest. WLT-based monitoring anonymously samples the location of probes or drivers by using wireless devices, such as cellular phones. By using a series of these probe vehicle locations, WLT-based monitoring systems could generate true point-to-point speed estimates with a minimal infrastructure investment. Unfortunately, early deployments of these systems have not been entirely successful in generating the quality of information needed by transportation agencies and the public. When transportation agencies are approached by vendors of WLT-based monitoring systems, they have little information to determine whether the system is likely to be effective. The relationship between the design of a WLT-based monitoring system and the accuracy of speed estimates that it generates is explored. A simulation-based approach was used to define general guidelines for different aspects of system design and roadway network characteristics. The type of map matching used, the frequency between position estimates, the error of position estimates, and roadway network geometry were shown to have a significant impact on generating accurate speed estimates from a WLT-based monitoring system.