Here’s a report on Gestures and Speech concerning the technological Evolution of Human-Machine-Interfaces in Cars.
Sometimes, like when one is driving a convertible or if it is too noisy, speech input can not be used. Also if one want to continuously zoom or scroll in a map, speech input is not the right way. Here, gesture input can be a good alternative to existing input devices.
Gestures correspond to a movement of individual limbs of the body and are used to communicate information. Moreover the recognition of gestures can easily be executed by humans as an unconscious process. Human beings can identify certain movements as gestures although they neither know the semantic nor the specific form of the gestures.