A trаnspоrtаtiоn study finds thаt lead-vehicle headway distance decreases rapidly at lоwer following-vehicle speeds and then decreases more gradually as speed continues to increase. This relationship represents a [answer1] trend in headway distance relative to following-vehicle speed. The trend appears to be [answer2] in nature and could be best represented by a [answer3] function. The least-squares method is commonly used to determine the degree of model fit to observed data. In general, the number of model parameters should be [answer4] to reduce data-collection costs, model complexity, and analysis time.