We love talking about road geometry, and when you combine that with an interchange design, things can get very interesting!

Drawing from our extensive experience on projects like Gateway Upgrade North, the M1/M3 Gateway Merge, and the M1 Pacific Motorway Upgrade North, we have gained valuable insights and learnings to share.

Intersections vs interchanges

Do you know the difference between an intersection and an interchange? An interchange is a grade-separated intersection that allows for the movement of traffic between two or more roadways, connected by ramps.

Introducing an interchange may alter the function of an existing road network, so a great deal of planning needs to occur before an interchange design can be detailed. Designers and engineers need to consider:

1. Traffic analysis
2. Spacing between adjacent interchanges
3. Continuity and consistency for road users
4. Land-use and lane management
5. Type and form of interchange
6. Ramp length and spacing
7. Delineation and signage.

Interchanges can be a costly element of our road systems, so incorporating flexibility in designs to cater for potential future changes where possible is critical, as described in Austroads Guide to Road Design (AGRD) Part 4C. Ultimately, interchanges improve safety and capacity for road users by removing conflict points between major traffic movements and can increase accessibility to our motorway networks.

Types of interchanges

Interchanges can come in many different forms. The appropriate type of interchange can maintain operational capacity during peak demand conditions and retains consistency and continuity of route for motorists.

According to AGRD Part 4C, interchanges broadly fall into two categories:

1. Service interchanges – connects a major and a minor road
2. System interchanges – connects two major roads.

System interchanges provide for free flow on all ramps, with all movements occurring as merges and diverges, however they are often large and complex so care should be taken when designing the layout to minimise the number of lane changes.

Service interchanges are characterised by ramps ending in at-grade intersections, which may be controlled by regulatory signs, traffic signals or roundabouts.

Ausroads Guide to Traffic Management (AGTM) Part 6 provides further guidance on the different types of interchanges, and the advantages and disadvantages of each.

Whilst traffic performance and safety are key considerations, many other factors may affect the type of interchange that is ultimately adopted. These include:

✅ functional classification of the roads

✅ importance of the intersection in the road network

✅ volume and characteristics of traffic to be accommodated

✅ need for ramp metering (now or in the future)

✅ desired level of service generally or for a particular movement

✅ topography

✅ land acquisition.

Types of Ramps

We now know that interchanges broadly fall into two categories: service and system interchanges. However, did you know that interchange types can also be characterised by the basic shapes of their ramps?

🔃 Diagonal ramps for all turning movements

🔃 Directional ramps for most direct right-turn connection between two roadways

🔃 Semi-directional ramps for one right-turn movement

🔃 Loop ramps for where traffic needs to make a 270° turn

🔃 Outer connector ramps for the most direct left-turn connection between two roadways.

The design of ramps and ramp terminals must be such that vehicles can safely enter or leave the major road. On exit ramps, traffic should be able to leave the major road without interfering with the existing traffic flow via long queues. For entry ramps, road users should be able to join the major road at the same operating speed as the through carriageway in a safe gap.

To ensure the ramps are operating safely and efficiently, we must consider:

🚘Sight distance (ASD, MGSD and SISD)

🚘Longitudinal gradient

🚘Spacing between terminals

🚘Number of lanes on each ramp

🚘Length of ramps for storage and deceleration / acceleration

🚘Auxiliary lanes that may be required on the major road

The design of interchanges is no small feat, and we could talk about them for hours! For all interchange-related information, refer to AGR Part 4C and AGTM Part 6.

Interchange geometry

Following the process outlined in AGRD Part 4C, we know we need to step through these detailed geometric elements to design our service or system interchange, comply with national standards, and ensure that the interchange performs efficiently:

1. Cross section
2. Design speed
3. Sight distance
4. Horizontal alignment
5. Vertical alignment.

An important final step is to check your aquaplaning potential once your horizontal and vertical geometry has been modelled to determine if any adjustments need to be made. Easy, right?

Let’s have a look at each requirement in a little more detail.

Cross section

Arguably the most fundamental part of your design process is confirming your cross section.

You can refer to AGRD Part 3 for fundamental cross-sectional elements such as lane and shoulder widths, however Part 4C details interchange-specific considerations such as number of lanes required on ramps, ramp lane widths, and clearance envelope to structures.

Design speed

Although this seems like an obvious step, determining and agreeing your design speed early can be a huge time-saver. Design speed has flow-on impacts to all your geometric elements, but also other traffic related elements such as level of service and spacing of driver 'decision points'.

Sight distances

Beyond confirming your stopping sight distance (SSD) requirements on major and minor roads, you now also need to confirm there is appropriate SSD at entry and exit ramps. This is so the driver has sufficient time to comprehend the location of the entry or exit and perform the merge or diverge manoeuvre safely. Other checks include safe intersection sight distance (SISD), approach sight distance (ASD) and minimum gap sight distance (MGSD) at adjacent intersections.

Horizontal alignment

Project boundaries often control your horizontal geometry, however it's important to consider as large a radius as practicable on both your major and minor roads if a straight alignment cannot be achieved, as this can help to improve sight distance and maintain consistent operating speeds.

Vertical alignment

In general, the major road should be designed to match the terrain and surrounds and minimise disruption to local development. It's good practice to design exit ramps on an uphill grade, and entry ramps on a downhill grade to assist road users with acceleration and deceleration.

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