Lowbridge

A lowbridge double-deck bus is a double-decker bus which has an asymmetric interior layout, enabling the overall height of the vehicle to be reduced compared to that of a conventional double-decker bus. The upper deck gangway is offset to one side of the vehicle, normally the offside (or driver's side), and is sunken into the lower deck passenger saloon. Low railway bridges and overpasses were the main reason that a reduced height was desired.

The lowbridge design was introduced and patented by Leyland in 1927 on their Titan TD1 chassis. Early examples were delivered to Glasgow Corporation amongst other operators. One of the Glasgow vehicles is preserved at the Scottish Vintage Bus Museum, Lathalmond, Fife.

A major disadvantage of this layout was the inconvenient seating layout, with four-abreast seats upstairs making it difficult for passengers to manoeuvre past each other if those farthest from the gangway needed to alight first. A second disadvantage was the restricted headroom for passengers on the offside of the lower deck, as a result of the encroachment of the upper deck gangway. It was often the case that passengers would bump their heads on it when standing up to alight.

At first, there was no viable alternative to the lowbridge design, apart from the use of single-decker bus. However, the lowbridge type started to become obsolete when low-height chassis were developed, which used a dropped-centre rear axle to enable the lower deck gangway to be lowered. This enabled a low-height vehicle to be built without the need for the cumbersome seating layout upstairs. The first such design was the Bristol Lodekka which was introduced by Bristol in 1949. It had a height of around 13 ft 6 in (4.11 m) compared to a typical height of around 14 ft 6 in (4.42 m) for a conventional highbridge double-decker. It was however only available to nationalised bus companies (Bristol itself being part of the state-owned sector at that time). Other low-height double-deckers included the Dennis Loline (a version of the Bristol Lodekka built under licence); the AEC Bridgemaster and Renown; and the Albion Lowlander (a low-height version of the Leyland Titan PD3). The rear-engined Daimler Fleetline and Bristol VR were also low-height chassis. Nonetheless, despite the advent of the low-height chassis, the last lowbridge double-decker was not built until 1968.

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