8.3.4 Service Interchanges
Service interchanges is a term given to an interchange between a major road (e.g. freeway) and a (relatively) minor road of lesser importance where traffic entering and leaving the minor road does so through at-grade intersections at the ramp terminal with the minor road. In many cases, the form of intersection at the minor road terminal will most appropriately be a roundabout but the individual circumstances at each location will dictate the final form of the intersection. In urban areas, the intersection will usually be signalised if a roundabout is unsuitable.
Service interchanges may have diagonal ramps or loop ramps (refer to AGTM Part 6 (Austroads 2013a)). The most common form of interchange is the diamond interchange, which may be a spread diamond or a closed diamond.
Spread diamond interchanges are usually associated with rural areas as land is relatively inexpensive and hence the terminals are located a significant distance from the centreline of the major road (e.g. 250 m). That is, the terminals are ‘spread’ apart by up to 500 m, depending on whether the major road is depressed below the natural surface. This approach is taken to minimise the earthworks and hence cost of constructing fill embankments whilst meeting other design requirements.
Spread diamond interchanges, therefore, typically have ramps with a horizontal curve beyond the exit nose from the major road to encourage drivers leaving the major road to reduce speed by about 20 km/h and may have a second curve to further reduce speed. It is important, however, that the immediate approach to the ramp terminal is straight with adequate sight lines above the formation.
Closed diamond interchanges are usually associated with urban sites where land is at a premium and expensive. The terminals are located relatively close to the overpass or underpass, hence the name. The ramp alignment is usually relatively straight.
The ramp length is a function of the relative levels of the major road, the natural ground and the terminals at the minor road, as well as the distance a terminal is from the major road centreline. It is also dependent on acceleration distance for entry ramps and deceleration distance and vehicle storage requirements for exit ramps.
Typical exit ramp length is 300 to 450 m with a minimum of 200 m past the exit nose. However, the length is a function of the type of interchange and Table 8.2 can be used as a starting point for preliminary design purposes.
|Trial exit ramp lengths(1) (m)|
|Minor road over major road||Major road over minor road|
|Closed diamond||200–300 (270)(2)||350–450|
|Spread diamond||250–400 (300)(2)|
- The length of the ramp is measured from the nose to the intersection with the minor road – the values in the table are rounded approximate values.
- The figures in brackets are suggested lengths as a first estimate of the nose location. These lengths should be increased by 30 m if the ramp is located on the outside of a superelevated curve.
On a flat entry ramp that is 300 m long an average truck can accelerate from a stop to about 50 km/h to 60 km/h and the speed decrement at the merge is excessive. It is often impractical to provide very long entry ramps and hence they should generally be in the range of 300 m to 450 m. As a general principle it is preferable to provide an auxiliary lane on the freeway at the entry rather than a longer ramp in order to provide a satisfactory distance for acceleration to occur and vehicles to merge with traffic on the major road (refer to AGRD Part 4A (Austroads 2010a) for acceleration distances for cars and guidance on treatment of trucks).
For ramps where there is a significant number of trucks and an uphill grade (e.g. in excess of 3%) the provision of two lanes on the ramp may be considered to allow light vehicles to overtake trucks (refer to Section 5.2).
Exit loop ramps
Where exit loops are used at service interchanges and the minor road passes over the major road, the exit ramp from the major road is located under the overpass at the abutment. In such cases:
- The ramp nose should be located in advance of the overpass structure so that it is not obscured by the bridge abutment (Figure 8.2). For cloverleaf interchanges, a collector‑distributor road should be used to provide for this.
- The ramp alignment should be straight for a distance past the nose and under the abutment to allow drivers to see an adequate length of curve on the loop (about 5° of field of view).
- The length of ramp between the nose and loop should be adequate to enable drivers to comfortably decelerate to the operating speed of the loop (refer to AGRD Part 4 (Austroads 2009a) for deceleration distances).
- An alternative treatment using a series of speed reduction curves to progressively reduce vehicle speeds should be considered (Appendix A).
- Spiral transition curves should not be used on the approaches to loop ramps as drivers approaching the loop tend to overestimate the safe speed of the curve because the spiral changes the driver’s perception of the approaching curve radius.
The general minimum radius for an exit ramp loop is 55 m, with an absolute minimum radius of 35 m, the larger value being preferred where it is necessary to use a minimum value for high‑speed freeway interchanges (refer to Table 6.1 for appropriate usage). Loops of radius greater than 80 m are not generally used since the faster speed around the loop is negated by the extra distance travelled, and the greater area of land that is required.
Source: Department of Main Roads (2005).
Department of Main Roads (2005) has been superseded and Figure 8.2 has not been carried forward into Department of Transport and Main Roads (2014).
Because of the significant difference in speed of the through road and the loop, drivers may approach the curve of the loop at too great a speed. Loss of control can occur and is most likely in the first 80 m of the loop. A run-out area should be provided around the outside of the loop as shown in Figure 8.2 and should be free of obstructions and hazards. If a driveable area cannot be provided as shown, a road safety barrier should be installed around the outside of the curve.
Where the major road passes over the minor road the exit loop may be on a downhill gradient and measures to slow the approach speed of vehicles are even more important. In this case the nose is likely to be a short distance beyond the abutment of the overpass with the ramp disappearing down to the driver’s left. Other cues and measures such as high fencing will be required to delineate the shape and location of the loop.
Entry loop ramps
For entry loops, the minimum radius should be 55 m but in urban areas in confined conditions, radii as low as 30 m can be used provided the maximum grade criteria in Table 9.3 are met. If the exit speed from the minor road exceeds 60 km/h, the layout of the approach to the loop should be similar to that in Figure 8.2 but adapted to the minor road.