Road geometry objectives

What is road geometry? Well, that's a bit of a loaded question, and an extremely layered topic. We will walk you through how we define road geometry at Marvel Engineers by breaking it down into several key elements. ‍

Austroads' Guide to Road Design defines Road Design as "the process of selecting and combining appropriate elements that will develop a fit-for-purpose solution. It is an iterative process that requires a designer to exercise their judgement and experience whilst also practically applying accepted technical guidelines and continually evaluating the design to assist in the selection of appropriate values for each design element".

This design process is governed by the following principles and objectives:

🎢 Geometric consistency
📱 Future technology considerations
🏆 Performance-based design
🏡 Community expectations

Roads will continue to be an important part of our transport system, providing safe and efficient movement of people and goods.

Here at Marvel, the fundamental principles and objectives of road design govern how we approach each project. By doing so, we can improve operational efficiency, road safety and public amenity, and minimise the effects of noise, vibration, pollution and visual intrusion. Seamlessly connecting infrastructure with people and nature.  

Cross section

Have you ever tried to fit a square peg in a round hole?

This is exactly why a road's cross section is the first, and arguably the most important, element to determine.

Firstly, we define the functional requirements of our road such as location, function, traffic volumes, speed parameters and the types of vehicles that will ultimately drive the road. These require endorsement from all stakeholders who play a role in construction, operations and maintenance of our road in the future.

Once agreed, the functional requirements inform the most appropriate values for the following fundamentals of our road cross section:

🚚 Traffic lane & shoulder widths
🚑 Verge widths & batter slopes
🚜 Median widths & treatments
🚲 Footpaths & cycle lanes 
🚅 Public Transport provisions

As our planet becomes more urbanised, we need to develop a deeper understanding of the people and environment the future road connects to when determining the values of our cross section fundamentals. We find this approach gets more out of the available space, leads to improved safety outcomes and reduces costs of road construction, maintenance and operations. We are creating greater value for local communities with less materials.

Horizontal geometry

If you're anything like us here at Marvel, you enjoy the feeling of centripetal forces at work as you glide around a perfectly designed horizontal curve... 

Which leads us into the second element of designing our road - horizontal geometry.

AGRD Part 3 emphasises that "generally, the adopted alignment should be as direct as possible, with curve radii as large as practicable." This is to permit speeds consistent with driver expectations on the curve itself and subsequent segments of the road, and to avoid sharp and unexpected changes in direction which could result in unsafe driver behaviour. 

Who would've thought psychology would play apart in road design!

When designing our horizontal geometry, it is important to understand adjacent hazards and constraints, as this will dictate the length of straights and even the type of horizontal curves we adopt. 

Did you know there are many different types of horizontal curves? These include:

🌀 Circular
🌀 Compound
🌀 Broken Back
🌀 Reverse
🌀 Spiral

No matter which type of horizontal curve we adopt, each one has an important element in common: super elevation. It tilts the carriageway around a curve to counteract side-friction, weight, velocity, centripetal & centrifugal forces that act on a vehicle. Without it, vehicles would overturn and roadways would see a lot more wear and tear due to unevenly distributed loads.

Vertical geometry

We're onto vertical geometry now and we have one mission: to make K values cool again.

Vertical alignments are "made up of a series of grades and vertical curves" and are "usually controlled toa large extent by features that the road passes through" as defined by AGRD Part 3. Vertical curves can be expressed by the constant 'K', which is used to define the size of each parabolic curve. 

Now that our cross section and horizontal alignment has been developed, it is time to consider the following fundaments which dictate our vertical alignment: 

🎢 Grading (slope and length)
🎢 Vertical curves (sags and crests)
🎢 Clearance to structures (new and existing)
🎢 Earthworks balance
🎢 Existing topographic features
🎢 Geotechnical conditions 

It is important to note that our horizontal alignment will most likely be adjusted to accommodate any constraints with the vertical. No discipline can be designed in isolation, and this balance of the two alignments is an important exercise to create the most value for our project. 

Sight distance

We've come to the final topic in our Road Geometry Explained series: Sight Distance.

Sight distance is detailed across two chapters of AGRD - Part 3 and Part 4. These chapters discuss each of the various sight distances that must be achieved depending on where we are on the carriageway and the proximity of upcoming intersections and crossings. These include:

👀 Stopping Sight Distance (SSD)
👀 Approach Sight Distance (ASD)
👀 Safe Intersection Sight Distance (SISD)
👀 Minimum Gap Sight Distance (MGSD)
👀 Crossing Sight Distance (CSD)

Each of these sight distance checks takes into account multiple parameters: operating speed of the vehicle, coefficient of deceleration, eye height of the driver, target height of the object, and driver reaction time. That's a lot of considerations when you are hitting the brakes! 

Sight distance also has a huge influence on the fundamental design of roads, intersections, roundabouts and interchanges. As AGRD Part 4A puts it: "The type and extent of sight distance available will significantly influence the design and location of an intersection. Both horizontal and vertical sight lines must be checked to ensure that they are not disrupted by natural objects such as trees, fences, buildings and safety barriers."

As you can see, road design is a complex activity. We must coordinate our cross section, horizontal geometry, vertical geometry and sight distances to provide a comprehensive and safe outcome for road users and our communities. 

Which 'Engineering Explained' topic should we dive into next?