This is draft guidance, and we welcome your feedback |
Essential general vertical, horizontal and underside clearance dimensions for public transport corridors.
For further clearance advice related to specific street elements such as trees, verandahs etc, refer to:
Vertical clearance requirements for single-deck and double-deck buses.
The key consideration for vertical clearance is the height of the vehicle and the type of obstruction. Although a shorter vertical clearance is acceptable for routes with only single-deck buses it is recommended that, where feasible, vertical clearances for double-deck buses be achieved on all routes to futureproof the network for potential fleet changes.
The vertical clearance for a bus to safely pass under obstacles depends on which type of bus(es) will use the infrastructure.
For corridors that only use single-deck buses, the minimum vertical clearance required is 3.65m to static obstacles such as verandahs and 3.85m to changeable obstacles such as trees.
The required safe vertical clearances applies to both rigid single-deck buses and articulated buses because both these bus types have the same overall height dimensions.
Additional clearance may be required if the bus has been modified and is not within the standard dimensions noted in the following section:
An example is electric buses which may have batteries located on the roof increasing the overall height of the bus.
For double-deck buses the recommended minimum vertical clearance is 4.6m to static obstacles (such as verandahs) and 4.8m to changeable obstacles (such as trees).
The extra safety margin for trees is to allow for changes from new growth and wind, snow and rain effects that occur in between inspections.
Additional clearance may be required if the bus has been modified and is not within the standard dimensions for the bus type. For example, corridors servicing double deck electric buses equipped with pantographs may require additional clearance.
Horizontal clearance is the amount of space that should be provided as a safety buffer between buses and roadside obstructions.
The key consideration for horizontal clearance is the front and tail swing of the bus which is affected by the type of vehicle and the road cross section.
Where a bus makes a tight turn or manoeuvre the front and rear corners of the vehicle rotate outwards past the path followed by the tyres.
Where the front of the bus swings out beyond the front tyres, this is known as front swing. Where the rear of the bus swings out beyond the rear tyres this is known as tail swing.
The potential overhang of the vehicle at a bus stop due to front swing and tail swing is shown below.
The amount of front swing and tail swing depends on how tight of a turn the driver makes and the design of the bus, such as the amount of overhang in front of or behind the axles. Some buses are fitted with steerable rear axles which assist drivers to make tight turns, but which also increase front swing and tail swing.
See 'Crossfall, road camber and superelevation' under Corridor constraints section
The potential for bus front swing and tail swing to extend over the footpath is an important safety consideration when designing bus stops. The amount of front swing and tail swing is influenced by the available length of lead in and lead out space either side of the bus stop box, the design and type of the bus and the road crossfall.
To prevent the front swing and tail swing of a bus from colliding with fixed obstacles a minimum horizontal clearance space of 1m measured out from the kerb line should be provided.
Where the bus stop is likely to be serviced by double deck buses, the 1m horizontal clearance should be extended 10m either side of the bus stop box, that is alongside the lead in and lead out space.
For information about the design of bus stops refer to the following:
A 0.5m horizontal clearance is required for bus corridors where the bus is travelling in the lane immediately adjacent to the kerb for example where no on street parking is provided.
This is to allow for the tilt of a double deck bus at up to a 5% crossfall which results in the bus extending almost 0.2m over the kerb, plus an allowance for a movement envelope to account for sway as the bus travels along the corridor.
The 0.5m horizontal clearance also provides a safety margin from bus front swing and tail swing which can occur during cornering.
Underside clearance provides adequate clearance for vertical deflection of buses at underside, front and rear overhangs is essential.
The majority of urban buses in New Zealand have a low-floor design which makes it easier for passengers to enter and exit the bus. However, the low-floor buses also have a lower underside clearance which means that bus bodies are closer to the road surface.
There is the potential for the underside of the bus to scrape the road surface negotiating heavily undulating road surfaces or traffic calming devices. Where the road gradient on a bus route exceeds 1 in 15 (6.7%) there should be a gradual transition in gradients to avoid sharp changes which could cause the bus to come into contact with the road surface.
When considering traffic calming designers should carefully consider the local street context and treatment selection, such as horizontal (eg chicanes) versus vertical deflection methods, as some treatments may cause:
Where horizontal deflection methods would not provide the desired traffic calming effect then raised tables and speed cushions may be used on bus routes.
Situations where raised tables may be appropriate include pedestrian crossings, intersections or low speed streets. Consideration should be given to using ‘one-way’ raised tables rather than traditional raised tables because these have a gentle exit gradient which provides a smoother ride for buses. Speed humps should not be installed on bus routes because the profile of these devices has a more severe effect on buses than cars.
Raised tables and speed cushions should not be placed near bus stops because of the increased risk of passengers losing their balance whilst moving around the bus preparing to alight. Speed cushions should be located on straight sections of the road to ensure that the bus can pass with the front and rear axles centred over the cushion.
Designers should also consider the cumulative impact of traffic calming devices on bus travel time and reliability and should consult with the public transport authority. The table below shows the recommended design guidance for vertical traffic calming devices more suited to bus routes.
Developed from: Auckland Transport, Engineering Code of Practice, Traffic Calming and Austroads, Guide to Traffic Management, Part 8 Local Street Management
Device type | Max ramp gradient | Max height (mm) | Min length (m) | Notes |
---|---|---|---|---|
'One way' (or median refuged) tables | 1:15(1:40 departure) | 75 | 3 | |
Raised tables | 1:20 | 75 | 6 | The top of the device should be at least the length of the wheelbase for the longest bus likely to use the road |
Speed cushions | 1:8 | 75 | 3 | 1.6m wide speed cushions are more suitable for bus routes |
For further guidance on traffic calming on bus routes, refer: