devising suitable control units a preferred system uses the multiplexing scheme described in u s pat no 4 907 222 the disclosure of which is incorporated herein by reference by way of example control unit 65 may comprise lo an intellitec pmc cpu 98a and two intellitec pmc i o modules edge 2 of the cassette 1 is provided with a resiliently biased flap 70 the resilient bias urges the flap to close slot 71 through which the first ramp 50 can be projected by motor 10 flap 70 serves a number of purposes flap 70 improves the aesthetic appearance of the vehicle flap 70 reduces the amount of dirt and water entering the cassette flap 70 reduces the height of the step between the platform of the vehicle and the ramp the top surface of ramp 50 is generally provided with a ribbed surface or other textured surface which may be resilient to improve traction of people and vehicles such as wheelchairs ascending or descending the ramp 50 second ramp 80 is received in the first ramp 50 when the ramp as a whole is retracted second ramp 80 is slidably movable relative to the first ramp in the illustrated embodiment this is achieved by providing slide guides 81 at the edges of the first in.