Sure you can. Nearest car is shown on the map and in my experience there was always something very close. Just choose "NOW" and it'll get booked. Besides, comparing pre-booking, it's done properly with AL, because it sends you a confirmation right away after driver accepts. With Uber the system would simply "schedule" your booking for the time specified and then fire request up as usual. Which means you're at a mercy of nearby driver's acceptance. And you have a false expectations that the pre-booking is more timely assured.