Suppose the speed of bus 18 is x m/min, the speed of Xiao Wang is y m/min, and the distance between two adjacent cars driving in the same direction is s m. 。

If a 18 bus passes behind every 6 minutes, then 6x-6y = s.①

Every 3 minutes, a 18 bus comes from the top, and then 3x+3y = s.②

S=4x can be obtained from ① and ②, so

s/x=4。

That is, the departure interval of bus terminal 18 is 4 minutes.