Based on reviews from trusted online sites, combined with information from Zipcar's website, it appears that Zipcar performs performs below average in terms of vehicle availability options when compared to other car subscription services.
Based on one review posted on consumeraffairs.com, it appears that Zipcar received poor ratings for vehicle availability options. See the full review here. Specifically, this reviewer pointed out the following:
The app which usually works fine wasn’t working. I called Jessica who reserved and charged me for a car. The car wasn’t there. We called: “unexplained”. Closest replacement? An hour away, in the wrong direction. I asked if Zipcar would comp the travel time and cost: nope. After cancelling the car I never picked up I received texts stating if we didn’t return the car... I had to make a third (in one day, about the same car, to an online company) customer service call. And was told “sorry”, and that a refund of the car I was charged for but didn’t get should be coming, “in 3-5 business days.” For our trouble, from Zipcar? ZIP.