Vertical curves

Vertical curves will be required where changes in gradient occur at sags and crests. Except where indicated in the note to Table 5, curve lengths should be either:

  • the sum ‘K’ x ‘A’, where ‘K’ is given in Table 5 and ‘A’ is the algebraic difference of the gradients expressed as a percentage; or
  • the ‘minimum length for appearance’ given in Table 5 whichever is higher.
Image
Example calculation of length of vertical curve

Figure 5: Example calculation of length of vertical curve

Shorter curve lengths may be acceptable where there are exceptional difficulties in achieving the length normally required.

Table 5: Vertical curves for all internal roads
85th percentile design speed (mph) Minimum length of vertical curve
  K Minimum length for appearance
30 (a) 6.5 30
25 (a) 4 25
20 3 20
15 2 20

a) Design speeds on new residential development roads should be restricted to 20mph or less. Please refer to Design Manual for Roads and Bridges for speeds above 30mph.

To avoid stretches of road where water gathers, do not apply the minimum length where “A” is less than five on any sag curve that results in a low point on the road.

Early discussions should be held with the council regarding large, flat sites to ensure that the vertical alignment is acceptable. It may be necessary to provide combined kerb and drainage units to ensure both an acceptable alignment and drainage of the highway.

For crests it may be necessary to increase the length of vertical curve derived to achieve the visibility distance as set out in Table 5 above.
 

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