We propose a systematic method for improving the response time of forward collision warning (FCW) on vehicles. First, a performance metric, called the warning lag time, is introduced. We use the warning lag time because its measurement is practical in real driving situations. Next, we discuss two ideas to improve this warning lag time, vertical and horizontal methods. The vertical method gives an additional warning, derived from the cause of a car crash, to a normal FCW system. The experiment showed that it can improve the warning lag time by an average of 0.31sec. compared with a traditional FCW system. The horizontal method uses distributed sensing among vehicles, which helps the vehicle see farther. It can also improve the warning lag time by an average of 1.08sec. compared with a single vehicle FCW.