You are driving a 100-foot twin trailer combination at 50 mph. The road is dry and the visibility is good. You should keep at least ____ seconds of space ahead of you.

The formula for following distance is one second per ten feet of vehicle length, plus an extra second if you are traveling over 40 mph. Therefore, for a 100-foot vehicle traveling at 50 mph, you should leave a following distance of 100/10 + 1 = 11 seconds.