Typically, I think of intelligence in terms of the control of perception.  A related, but different, definition of intelligence is the (at least partial) restriction of possible future states. For example, an intelligent Chess player is one whose future rarely includes 'lost at chess to a weaker opponent' states; they're able to make changes that move those states to 'won at chess' states.
These are both broad and continuous definitions of intelligence, where we can talk about differences of degree. A sundial doesn't exert any control over its environment; it passively casts a shadow, and so doesn't have intelligence worth speaking of. A thermostat attached to a heating or cooling system, on the other hand, does exert control over its environment, trying to keep the temperature of its sensor within some preferred range. So a thermostat does have intelligence, but not very much.
Self-driving cars obviously fit those definitions of intelligence.
 Control is meant in the context of control theory, a branch of engineering that deals with dynamical systems that perceive some fact about the external world and also have a way by which they change that fact. When perception is explicitly contrasted to observations, it typically refers to an abstract feature of observations (you observe the intensity of light from individual pixels, you perceive the apple that they represent) but here I mean it as a superset that includes observation. The thermostat is a dynamical system that perceives temperature and acts to exert pressure on the temperature it perceives.
(There's a philosophical point here that the thermostat cares directly about its sensor reading, not whatever the temperature "actually" is. I think that's not something that should be included in intelligence, and should deserve a name of its own, because understanding the difference between perception and reality and seeking to make sure one's perceptions are accurate to reality is another thing that seems partially independent of intelligence.)