Express buses & traffic

[diceq]

How affected by traffic are the Expressways? For example, according to google, this route (bus 138, from Phaya Thai to Rat Burana):

http://goo.gl/maps/iaEa4

takes 18 minutes. Roughly, what would be the real average travel time if I were to travel that route in mornings between 7-8 and evenings 5-6?

Edited September 10, 2013 by diceq