This chapter addresses the reasons for non-compliance and explains how it is related to the precise phrasing of the Variable message signs (VMS) message. Collection of information on drivers' destinations can be achieved more simply by means of roadside interviews downstream of the VMS site. Visibility is a function of siting and size of the VMS panel, whereas legibility is a function of the size, spacing, colour and luminance of the lettering or symbols. Some VMS systems express conditions in terms of the length of queues rather than the length of delays. Models of response to VMS are required at two levels: firstly, to help evaluate a proposed investment in VMS by estimating the benefits which might accrue from different levels of provision; and secondly, to help, perhaps in real-time, with the selection of messages to be displayed at a particular site in a particular set of circumstances.