For many roadway applications, high-accuracy in-lane level vehicle position information is desired. Unfortunately, in many roadway environments GPS dead zones hinder sufficient GPS position accuracy. These roadway environments include underpasses, tree canopies, urban canyons, and any other locations where the view to the sky is limited. This report introduces a high-accuracy position estimator that augments GPS in areas where short-term (< 200 meter and < 15 second) GPS dead zones exist. The position estimator fuses differential GPS (DGPS) position measurements, yaw rate measurements, and two-dimensional velocity measurements to provide in-lane level accuracy position estimates. The estimator increases the availability of high-accuracy position estimates for applications that demand continuous high-accuracy in-lane level positioning, such as lane departure warning systems. The position estimator was evaluated and the position accuracy was quantified. Seven vehicles were outfitted with the position estimator system. Data was collected for 460 DGPS outages and the accuracy of the system was analyzed. From the analysis the position accuracy of the estimator could be approximated based on the distance and time since the DGPS outage began. This analysis provides a level of confidence in the position estimates as a function of distance and time elapsed from the start of a DGPS outage. This level of confidence measure allows applications to have a means to reject position estimates based on the outage time and distance if those estimates are projected to have lower accuracy than the application requires.