Pending that revolution, tramway authorities are seeking to develop a cheaper means of electric traction than the tramway. At the present stage, urban tramways have spread through suburbs towards villages and small towns which are anxious for better transport facilities but have not sufficient population to justify a tramway extension. Inter-urban tramway systems--those connecting towns with a network of lines--are also adjacent to such minor centres of traffic. From time to time attempts have been made to meet the demand by means of petrol omnibuses, but they have rarely been successful--partly, no doubt, owing to the difficulty of working a limited petrol omnibus service economically at the extremities of an electric tramway system. The latest solution of the problem is the 'trackless trolley' or, more correctly, the 'trolley omnibus.' In the 1911 session over a dozen tramway authorities applied for powers to use this device; and, if the financial results of the first attempts are successful, there will probably be a considerable growth in this type of electric traction. The trolley omnibus is a hybrid between the trolley tramcar and the omnibus. It is akin to the first, because it derives its power from an overhead wire through a flexible trolley pole. It is akin to the second, because it does not run on rails but is fitted with solid rubber tyres and uses the surface of the road in the usual way. Roughly speaking, its electrical equipment is similar to that of a tramcar. The trolley pole conveys the electric current to the controller, which admits it to motors geared on to the back axles. There are, however, one or two important differences. The absence of a rail which might act as a return conductor necessitates the provision of a second overhead wire and a second trolley-pole to connect with it. Thus the electrical circuit is from the power station, along the first overhead wire, down the first trolley-pole, through the controller and motors, up the second trolley-pole, and back by the second overhead wire to the power station. Owing to the vehicle being a steerable one, the trolley-poles have to be specially designed to give plenty of free play sideways. The vehicle itself is similar in appearance to a single-decked motor omnibus, and it runs on solid rubber tyres or spring wheels.