Lane departure warning (LDW) systems can detect an impending road departure and deliver an alert to allow the driver to steer back to the lane. LDW has great potential to reduce the number of road departure crashes, but the effectiveness is highly dependent upon driver acceptance. If the driver perceives there is little danger after receiving an alert, the driver may become annoyed and deactivate the system. Most current LDW systems rely heavily upon distance to lane boundary (DTLB) in the decision to deliver an alert. There is early evidence that in normal driving DTLB may be only one of a host of other cues which drivers use in lane keeping and in their perception of lane departure risk. A more effective threshold for LDW could potentially be delivered if there was a better understanding of this normal lane keeping behavior. The objective of this paper is to investigate the lane keeping behavior of drivers in normal driving. The study will be based upon data extracted from the Integrated Vehicle-Based Safety Systems (IVBSS) Naturalistic Driving Study conducted by University of Michigan Transportation Research Institute (UMTRI). The study first presents the distributions of DTLB and lateral velocity during normal lane keeping and then examines the relationship between DTLB and lateral velocity as a function of lane width and radius of curvature.