I cancelled the taxi; must I pay anyway?
A customer who orders a taxi, and then cancels in advance with legitimate cause, is not required to pay.
On the other hand, if the driver lost another potential job because of the reservation, the customer is liable. The same applies if the driver already began traveling to the pick-up location and was unable to recoup his fare by engaging another customer.
The sum required of the customer is not full payment for the job, but an amount k’poel batel (like an idle worker), i.e. the going rate that a hiree would accept for “sitting on the job” within that time frame. This amount can be calculated at about half of a regular fare, after operating costs were deducted (unless there were any expenses incurred for this particular job, which can be added). However, in places where there is a minhag hamedinah (local custom) for compensation in such situations, that figure should be the pay standard.
A customer who hires a taxi, and then encounters an unreasonable delay in its arrival, may cancel without liability.