=+4.44 The paper “Modeling and Measurements of Bus Service Reliability” (Transportation Research [1978]: 253–

256) studied various aspects of bus service and presented data on travel times from several different routes. The accompanying frequency distribution is for bus travel times from origin to destination on one particular route in Chicago during peak morning traffic periods:

Relative Class Frequency Frequency 15 to ,16 4 .02 16 to ,17 0 .00 17 to ,18 26 .13 18 to ,19 99 .49 19 to ,20 36 .18 20 to ,21 8 .04 21 to ,22 12 .06 22 to ,23 0 .00 23 to ,24 0 .00 24 to ,25 0 .00 25 to ,26 16 .08