6.4.1 Ramp Design Speed
The design speed adopted for ramps depends on the form of the interchange. For system interchanges between freeways the design speeds for ramps are generally higher than other forms and may be the same as the main carriageways.
There are two distinct types of ramps to consider when determining the operating speed on the ramp:
- The first type is where both terminals of the ramps are either diverges or merges, e.g. direct, semi-direct, outer connectors and some loops.
- The second type is where the ramp connects a major road to a minor road with one terminal being an intersection, e.g. diagonal ramp.
The design speed on both types of ramps at any point should be equal to the operating speed (85th percentile speed) on the ramp. However, the methods of determining the operating speed on these types of ramps are quite different.
For ramps of the first type, the operating speed is the lower value of the following:
- that predicted by using the operating speed model in AGRD Part 3 (Austroads 2010b)
- 10 km/h over the ramp posted speed limit.
The suggested minimum design speeds for these ramp types as a function of the desired operating speeds on the major road are shown in Table 6.1. It is most important to provide sufficient deceleration distance for drivers approaching at the major road operating speed to slow to the ramp speeds shown in Table 6.1 (e.g. to negotiate a loop in a parclo1 layout).
|Form of interchange||Ramp type||Design speed of through road|
|70 km/h||90 km/h||110 km/h||120 km/h|
|Ramp design speed(1)(km/h)||Minimum ramp radius(2) (m)||Ramp design speed(1)(km/h)||Minimum ramp radius(2)(m)||Ramp design speed(1)(km/h)||Minimum ramp radius(2)(m)||Ramp design speed(1)(km/h)||Minimum ramp radius(2)(m)|
|System interchange||Direct||Not applicable||70–85||130–250||85–100||250–440||90–110||330–580|
- Ramp design speed ranges derived from operating speed model relative to minimum ramp radius.
- Minimum ramp radius derived from AASHTO (1994).
A parclo interchange layout is in the form of a partial cloverleaf.↩