Road design

4.6 Preparing and Checking Designs

The following steps are recommended to prepare and check a design layout.

  1. Select the appropriate design vehicle as discussed in Section 4.1.4.
  2. Select the template with the radius of turn matching (as near as practicable) the turn required, bearing in mind the geometric controls of the site and the most likely vehicle turning speed. The use of the absolute minimum turn radius should be avoided on arterial roads and at other important traffic sites. Each of the templates indicates a range of appropriate speed values for the radius of turn chosen.
  3. Correctly align the template within the lane from which the turn is to be made. Note that the templates are oriented for left turns.
  4. Note the angle of turn required and rotate the template, about the centre of the turning radius until the nearest greater angle given on the template matches the exit direction. (This requires rotation of the template in the opposite direction to that of the vehicle movement).
  5. Mark/note the vehicle’s wheelpath.
  6. If a reverse curve is involved (e.g. as in the layout of a roundabout), particular care needs to be taken in allowing for the transition of the wheelpath from one direction to the other. Considerable skill is required to correctly achieve this with the use of conventional single radius uni-directional turning path templates. In critical situations, designers are encouraged to use available computer-based turning path prediction software models which permit the swept path and wheelpaths to be plotted for a design vehicle following a complex turn path made up of arcs and tangents.
  7. Design vehicle turning path templates should be applied to road intersection layouts to accommodate the swept path with a minimum offset of 0.5 m from the extremities of the vehicle path to a kerb, pavement edge or centreline. Note that experience and engineering judgement should be used when applying clearances.  

    The 0.5 m offset need not be provided for local streets in urban areas where space is restricted or local access/minor roads in rural areas where the shoulder is partly sealed. However, it is desirable that the vehicle (i.e. swept path) should not cross the centreline of a minor rural road. A minimum clearance of 0.5 m outside the swept path should be provided to objects such as road furniture and utility poles.

    In situations where space is restricted and turning speed is low, it may be necessary to allow the swept path of the design vehicle to encroach into a verge or traffic island with the wheelpaths remaining on the pavement. This may occur when designing for a large bus to use a local street, or when checking that a design layout can accommodate an occasional vehicle larger than the design vehicle. The long rigid bus may be appropriate for checking designs on arterial roads.

Where computer programs are not available, the designer may graphically check a layout design that involves a reverse turn by the following method: use the appropriate template to plot a trace, by hand, of both the outer front wheelpath and the inner path for the design vehicle, in the direction of the first turn. The reverse of the template should then be used to check the opposite direction of the turn by overlaying the template and aligning the wheelpaths previously traced. The template should be aligned so that the inner path on the template is tangential to the outer front wheelpath already plotted and, simultaneously, the outer front wheelpath is tangential to the inner path already plotted. The template will need to be rotated in order to maintain this requirement and to fit within the design controls at the site.

While this procedure may not exactly reflect the actual positioning of the vehicle as it moves through the reverse curve, experience has shown that the wheelpaths plotted using this technique match sufficiently the actual wheelpath plots produced using one of the computer-based turning path models.