There are several methods to discover travel demand. Such can be the use of questionnaires or the application of a “check in – check out” e-ticketing system.
The application of an e-ticketing system can collect very detailed and accurate time dependent data day by day. But the establishment of such a system is very expensive and for a small bus operator unrealistic.
Another possible way is to organise questioners, but this costs a lot of financial resources and needs a large number of employees. Furthermore, the accuracy and reliability of the data are not always perfect, because reliable data need a large sample several times a day.
As an example, we can see a Hungarian city with 30,000 trips /day. This means about 7,000 trips in the morning peak period. The city can be divided into 25 zones, which means 625 possible trip relations. Some of these possibilities are not realistic or are used by only a few passengers; therefore, the number of real transport connections are about 100. If it is needed to take a sample from the morning peak, we have a basis of 7,000 persons. We can discover only the relations with at least 70 passengers, which means the incidence ratio is P=0,01. With the use of the common reliability of 95% and 10% relative error, the sample size is as follows (Eq. (1)).