2.4.2 Interchange Uniformity and Spacing
Route considerations are discussed in Section 6.3 of AGTM Part 6 (Austroads 2013a). Important aspects from a route perspective are the spacing of interchanges, route continuity and consistency in the form of interchanges along a route.
With respect to route continuity it is necessary to provide a design that favours major movements as the designated through route/s and the route and interchanges should have a form of design and signage to reflect this driver expectation.
Drivers can become confused if the form of interchange and exit conditions change repeatedly along a route. Driver perception of ease of negotiating interchanges from both the major and minor roads is an important factor in efficiency of operation and the safety of the network. While interchanges should be custom-designed to suit the specific conditions of the site, consistency can be achieved along a link through the use of a consistent form of interchange, but it is also achieved by a consistent approach to the pattern of the exits and entrances and their signing.
For example, drivers expect to exit to the left and they expect the ramp to start in advance of the separation structure. If this feature is incorporated regardless of the form of the interchange beyond the exit, consistency will have been achieved. A similar approach to entrance ramps should be taken.
This is particularly so in rural areas where drivers may generally expect diamond interchanges, although semi-directional (free-flow) interchanges are often used for access to towns that are being bypassed. Where these interchanges are used designers should not produce a design that accommodates high-speed ramps and local crossing and access movements within the one interchange as this invariably leads to driver confusion and road safety issues.
Where a system interchange is being implemented, local access connections should be avoided. Mixing the local access arrangements and the requirements for free flow in the system interchange leads to inconsistency of form and a lower level of road safety in the interchange. Local access arrangements should be accomplished at an appropriate service interchange.
The location of interchanges is usually determined by the road network requirements for accessibility and route interconnectivity. However, there are limits to the number of interchanges that can be accommodated and to the spacing of the ramp terminals without compromising the capacity and safety of the road.
Every ramp, whether an entry or exit ramp, creates conflict and causes some disturbance to the traffic flow on the freeway. The effects of this disturbance are felt for some distance on each side of the ramp/carriageway terminal. Examination of these effects gives a clear indication of the minimum spacing that can be tolerated (refer to AGTM Part 6 (Austroads 2013a), Section 6.6.6 and Table 6.3).
The spacing of interchanges is important because it can result in issues associated with the overlapping or insufficient separation of entry ramps and exit ramps. This in turn can lead to the use of an inconsistent form of interchange or difficulties in locating major direction signs in a satisfactory location. More importantly, it may result in problems associated with traffic weaving between interchanges. However, this does not take account of other factors that influence interchange spacing such as:
- Signing requirements (refer to AS 1742.15; MOTSAM Part 3, (NZ Transport Agency (NZTA) 2010b) and jurisdictional guides).
- The distance required for a driver to change lanes to position the vehicle sufficiently in advance of an exit to safely undertake the manoeuvre. Buses and heavy vehicles require 200 m per lane change, while cars desirably require 150 m per lane change1.
- The extent of weaving caused by the placement of successive entry and exit ramps. Weaving introduces an additional element of conflict and has a negative effect on both levels of service and safety. TRB (2010) provides details of the methodology required to address the weaving issue from a level of service point of view (refer to AGTM Part 6 (Austroads 2013a), Section 6.6.6 and Table 6.3).
The general conclusion that can be drawn from these requirements is that the minimum spacing of interchanges is:
- in urban areas about:
- 2 km on four-lane freeways (i.e. two lanes in each direction)
- 3 km on six-lane freeways
- 4 km on eight-lane freeways
- in rural areas between 5 km and 8 km.
It is emphasised that these minimum spacing for urban areas are not necessarily desirable and should be checked to ensure that the main carriageways will operate satisfactorily. Traffic analysis may indicate that the desirable spacing is greater than these values. It follows that the ultimate number of lanes should be considered when the location of interchanges is initially planned.
Where other factors dictate the need to have the interchanges at closer spacing than these, the required spacing can be effectively achieved through the form of the interchange. For example, the ramps can be grade separated (‘braided’ ramps) to create a bigger spacing of the ramp terminals or collector-distributor roads can be used with the added advantage of keeping local traffic clear of the main through traffic. Different forms of interchange and their relative advantages and disadvantages are provided in AGTM Part 6 (Austroads 2013a).
Maximum spacing is less easily determined and will depend on the needs for accessibility and service to the local road network. In urban areas, spacing above about 4 km would not be expected. In rural areas a spacing greater than about 12 km should be carefully examined for adequacy of service and where spacing above this is proposed, the overall level of service provided by the road system should be reviewed.
Where long lengths of rural freeway do not need interchanges for access and service reasons, the need for rest areas and/or service centres has to be assessed to ensure that drivers have adequate facilities for rest, refreshment and refuelling. The need for U-turn facilities should also be considered.
The merge distances are based on a rate of lateral movement of 0.6 m/s for cars and a lower rate of 0.5 m/s adopted for buses and heavy vehicles. Note: distances have been rounded.↩