Is the specification of the travel route's duration a necessary condition for contract validity when no established custom exists?

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If the route lacks an established custom ('Urf) and the parties conclude the contract without specifying the travel measure, the contract is invalid according to Al-Qadi, mirroring the rule for an unspecified price in a town lacking customary currency. However, the preferred view is that specifying the travel duration is not a condition (Shart) for validity. This is supported by the fact that the contract would remain valid without specification even on a dangerous route, and custom does not typically mandate specifying the daily travel measure on every route. In case of dispute where custom is absent for that specific route, custom from similar routes should be consulted.