Externalities are the (marginal) costs that a user of a common resourc
e imposes on others. We introduce the efficient measure of tangible ex
ternalities that are the costs that a user imposes on others while bei
ng served. Then, for a single sen er queueing system under various ser
vice disciplines, we compute the expected externalities and the expect
ed tangible externalities conditioning on the length of the service re
quirement.