With high validity naturalistic driving data, this study investigated the distribution of drivers’ car-following headways and how drivers would adjust their headways due to level of operating speed, visibility, roadway type, traffic density, and the intervention of a Forward Collision Warning (FCW) system. From 60,689 km of naturalistic driving data, 1,489 car-following events were identified. Headways were then extracted and statistically compared across different driving situations to quantify changes in car-following headways as a result driving situations. The results of this study show that (1) the distribution of car-following headway was similar to a lognormal distribution; (2) drivers tended to maintain longer headways in slow-speed driving, nighttime, surface roads, dense traffic conditions, and when FCW activated.