I don't have the case, but based on the info provided:
1. Opex are given as % of revenues (e.g., opex = 60% of revenues), thus km*$/km* vehicles are the revenues, and multiplying it by 1-opex (which is the percentage of revenues - 100% - minus the percentage of costs, meaning the percentage of revenues that is profit), gives you the profit
2. km*$/km* vehicles are exactly the concession company revenues. Basically, when you use the highway, you pay an amount that depends (mainly) on the kilometers of the highway you "used" at that is true for all cars, trucks, and motorcycles (with different fares per vehicle). This happens every time a vehicle enters the highway. This is not true for all countries (e.g., in Switzerland you pay an annual tax regardless of the kilometers or the time you use the highway), but for most of them it is. Probably in this case, they wanted to simply the revenues (which of course depends also on where you entered and where you exit from the highway + other revenue streams, like concessions for additional services, like drive-in, restaurants, oil stations, and so on) as a revenue per kilometer per vehicle.
Hope this helps!