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Unit 1: Introduction to System Simulation

project could be to obtain performance data that would assist in determining whether such a Notes
policy would improve the flow of vehicles through the station. Vehicles are obliged (via
appropriate signage) to access the pumps from the east side and after their respective gas purchases
they exit on the west side. Upon arrival, drivers always choose the shortest queue. In the case
where two or more queues have the same shortest length, a random choice is made. An exception
is when it is observed that a customer in one of the ‘shortest queues’ is in the payment phase of
the transaction in which case that queue is selected by the arriving driver.
Figure 1.4: Gas Station Project

Depending on the time of day, one or two attendants are available to serve the customers. The
service activity has three phases. During the first, the attendant determines the customer’s
requirement and begins the pumping of gas (the pumps have a preset delivery amount and
automatically shut off when the preset amount has been delivered). In addition, any peripheral
service such as cleaning of windshields and checking oil levels are carried out during this first
phase. Phase two is the delivery phase during which the gas is pumped into the customer’s gas
tank. Phase three is the payment phase; the attendant accepts payment either in the form of cash
or credit card. The duration of phase two is reasonably long and an attendant typically has
sufficient time either to begin serving a newly arrived customer or to return to handle the phase
three (payment) activity for a customer whose gas delivery is complete. The protocol is to give
priority to a payment function before serving a newly arrived customer. It is standard practice
for the payment function to be carried out by the same attendant who initiated the transaction.
The above text can be regarded as an initial phase in the model building process for this particular
modelling and simulation project. Notice, however, that much detail remains to be added; for
example, the specification of the arrival rate of vehicles, the proportion of vehicles that fall into
the small truck/van category, service times for each of the three service phases, and so on (these
correspond to data requirements). Nor should it be assumed that it is entirely complete and
adequately comprehensive.
Refinements to this description are almost certain to be necessary; these may simply provide
clarification (what are the conditions that govern the attendant’s options during phase two) or
may introduce additional detail; such as what happens when a pump becomes defective or,
under what conditions does an arriving customer ‘balk,’ that is, decide the queues are too long
and leave. Or, in fact, is balking even a relevant occurrence? What about accommodating the
possibility that drivers (or passengers) may need to use the washroom facilities and thereby
‘hold’ the pump position longer than is otherwise necessary? The merits of introducing such
refinements must always be weighed against their relevance in terms of achieving the goals of
the modelling and simulation project. (It may be useful for the reader to dwell on other possible

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