A set of values used to specify the mode of travel. NOTE: WALK, BICYCLE, and TWO_WHEELER routes are in beta and might sometimes be missing clear sidewalks, pedestrian paths, or bicycling paths. You must display this warning to the user for all walking, bicycling, and two-wheel routes that you display in your app.

TRAVEL_MODE_UNSPECIFIED No travel mode specified. Defaults to DRIVE.
DRIVE Travel by passenger car.
BICYCLE Travel by bicycle.
WALK Travel by walking.
TWO_WHEELER Two-wheeled, motorized vehicle. For example, motorcycle. Note that this differs from the BICYCLE travel mode which covers human-powered mode.
TRANSIT Travel by public transit routes, where available.