As far as I'm currently aware, the Falcon 9 block 5 does not have the capability to retract it's landing legs once they're deployed, however:
If F9 landing legs had the ability to retract themselves (as Elon has mentioned is a goal for full reusability), would the Falcon 9 be able to take off from any reasonably solid surface such as a parking lot or highway?
- Would the aerodynamics of unfolded legs interrupt the early stages of flight (until the legs are folded in)?
- Would the exhaust from the engines damage the underside of the first stage or the landing legs in such a way that it would be unable to take off or land later?
- Would the surface loose integrity and topple the F9 before it could take off?
- Or can the Merlins start up fast enough to avoid this problem?