On driving 100-foot truck with double trailers at 30 mph with the dry road and nice visibility, you should keep at least how many seconds of space ahead of you?

How much space should you keep in front of you? One good rule says you need at least 1 second for each 10 feet of vehicle length at speeds below 40 mph. At greater speeds, you must add 1 second for safety. [California Commercial Driver Handbook]