A 3 x 3 metre truncation provides Safe Intersection Sight Distance (Austroads Guide to Traffic Engineering Practice: Part 5) at 4-way intersections and T-junctions, other than where the intersecting road is on the inside of a small radius curve. In the latter case, the provision of a
Appendix B – Technical Investigations
Page 48
larger truncation doesn’t improve sight distance, as the sight line is
‘through’ the block, not ‘across’ the corner.
A 3 x 3 metre truncation provides flexibility for services, particularly stormwater drainage. It is often preferable, or even necessary, for stormwater pipes to be laid diagonally across the corner. Other services also benefit from the presence of a 3 x 3 metre corner truncation.
The 3 x 3 truncation is also provided to give some improved corner sight distance for footpath users, particularly as young cyclists will be permitted to ride on footpaths when the new Australian Road Rules are implemented in Western Australia.
‘Visual’ truncations alone (i.e. reduced wall height at corners) will not be adequate on their own and will probably attract community resistance, especially small lot subdivisions with small front ‘courtyards’. Reduced wall height may still be pursued as an additional requirement to the 3 x 3 metre truncation for those Councils seeking a more conservative approach to corner sight distance.
Excluding those ‘special geometric’ cases noted above, the 3 x 3 metre truncation will be adequate at all local street intersections (e.g. AS/AS, AS/NC) which do not employ roundabout control. Most NC/NC intersections should be designed to cater for roundabout control due to the high probability that they will be warranted on traffic volume and speed control grounds at the planning stage. For roundabout control, corner truncations should be determined on a case by case basis as part of the intersection design.
Appendix B – Technical Investigations
Page 49
Issue 2: Kerb Return Radius
Position
Provide a 6 metre kerb return radius as the default at AS/AS intersections (i.e. the intersection of two access streets). This caters for the B99 design car (turning radius 6 metres) using the correct side of the pavement only (Figure B1 in AS 2890.1, 1993). It also caters for the design heavy rigid vehicle (turning path radius 11 metres), using any part of the pavement (Figure B4 AS 2890.2, 1989).
Provide a 9 metre kerb return radius at Access Street / Neighbourhood Connector intersections. This caters for the design heavy rigid vehicle (turning path radius 11 metres), using any part of the pavement (Figure B4 in AS 2890.2, 1989). Kerb radii at NC/NC intersections (i.e. the intersection of two neighbourhood connectors) will depend on roundabout design requirements but should be kept to the minimum necessary to control speed of left turns which often pose problems for pedestrians.
Intersections along arterial routes should be designed to cater for articulated vehicles.
Comment
A kerb return radius larger than 9 metres is required at some intersections along Neighbourhood Connectors (i.e. some Access Street / Neighbourhood Connector intersections) due to the influence of the median on some Neighbourhood Connectors (volume >3,000 vpd) which restricts the width of pavement available for the turn.
Although rare on Access Streets, intersections where splitter islands or barrier lines are installed will also present problems for trucks using ‘any part of the pavement’, and will need a kerb return radius larger than 6 metres. These will need to be designed to suit.
There are implications for on-street parking on 5.5–6.0 metre and 7.2 metre carriageways, when heavy rigid design vehicles need to ‘use any part of the pavement’ when turning. Some turning manoeuvres cannot be achieved in a ‘single forward movement’ in cases where a vehicle is legally parked 10 metres from the intersection (ARR Regulation 170).
Parking along the continuing leg of a T-junction ‘within’ the intersection also restricts truck turning movements. Increasing the 10 metre minimum to 15 metres provides room for trucks to turn. We recommend accepting this situation because problems should be rare and the consequences of the rare situation being encountered are limited; the heavy vehicle will merely reverse and make the turn on the second attempt.
The potential for broken or damaged kerbs at corners should be monitored and concrete backing should be used where required.
Appendix B – Technical Investigations
Page 50
Issue 3: Treatment of 4-Way Intersections
Position
Arterial/arterial 4-way intersections to be signal or roundabout controlled.
Arterial / Neighbourhood Connector 4-way intersections to be signal or roundabout controlled.
NC/NC 4-way intersections intersections (i.e. the intersection of two neighbourhood connectors) to be roundabout controlled (10–12 metre inner island diameter) with adequate vehicle path deflection (refer to Austroads Guide to Traffic Engineering Practice: Part 6) to keep speeds low.
Neighbourhood Connector / Access Street 4-way intersections should be minimised by altering street block layout, particularly those with a strong desire line across the major leg.
AS/AS 4-way intersections (i.e. the intersection of two access streets) to be priority controlled via stop/give way signs. The occasional roundabout needed at AS/AS 4-way intersections need not cater for buses and should be small (i.e. 6–8 metre diameter) to reduce land take.
Comment
Small diameter roundabouts (approximately 10–12 metre diameter inner island) are common in Perth on bus routes. These appear an appropriate design for NC/NC intersections. Larger radii require substantial land take, involve additional construction cost, and pose added pedestrian and cycle safety problems due to higher vehicle speeds and longer crossing distances.
The Main Roads standard drawing requires a 12 metre diameter inner island for roundabouts on bus routes. Where appropriate a smaller diameter is favoured due to less land requirement and ease of pedestrian and cyclist use. Some Neighbourhood Connectors will, however, be designed with medians and these will require a 12 metre diameter inner island or larger to create the necessary vehicle path deflection through the intersection.
Appendix B – Technical Investigations
Page 51
Issue 4: Access Street Width
Position
Choice of Access Street width is related to the traffic function and the on- street parking demand. The choice of street width should be indicated on a street block layout plan (1 : 5,000 or greater detail) which also indicates the intended residential density, home-based business zoning, commercial or industrial zoning.