I am not at all convinced that improvisation is an easy add-on. While AI has demonstrated some creative ways to satisfy goals that have left options open (sometimes inadvertently), real-world improvisation has to be appropriate for the situation and the unusual difficulties it poses, and, in the case of driving, it must not create additional problems or (of particular relevance here) present significant dangers.
In general, I think, improvisation requires causal reasoning. It also often needs a broader knowledge than that needed for the nominal task (for example, when driving on a potholed road, one needs to have some understanding of how it affects the car's response, and sometimes of issues such as ground clearance.) Constraining the vehicle to avoid such issues would constrain the option to improvise.