This paper overcomes the long-standing problem of measuring motor carr
iers' service quality by utilizing service quality data collected from
shippers. A service quality controlled cost function is estimated for
a sample of LTL motor carriers. It is shown that excluding service qu
ality variables from cost estimation significantly underestimates carr
iers' scale economies. These scale economies enable large carriers to
offer higher quality services at competitive or even lower costs than
small carriers.