1. A computer-implemented method of controlling a vehicle, comprising: receiving one or more instructions issued as speech; analyzing the speech using speech recognition software to provide a sequence of words and a word confidence measure for each word so recognized; analyzing the sequence of words to identify a semantic concept corresponding to an instruction based on the analysis, and a semantic confidence level for the identified semantic concept derived at least in part with reference to the word confidence measures of the words associated with the semantic concept; providing a spoken confirmation of the semantic concept so identified based on the semantic confidence level and an indicated verbosity level, the spoken confirmation being provided with one of a speaking rate or a pitch that is increased as the indicated verbosity level decreases; and using the semantic concept so identified to provide a control input for the vehicle; wherein the step of providing the spoken confirmation of the semantic concept comprises indicating that the instruction was not understood when the semantic confidence level is below a threshold, or the step of using the semantic concept comprises providing the control input for the vehicle when the semantic confidence level exceeds the threshold.