Vertical Alignment Design: I Calculated Manually in School—How is Efficiency Achieved in Practice?

This series is part of a personal project aimed at consolidating what I’ve learned in school while diving deeper into the complex, real-world aspects of road design. As a future Draftsperson, I’m using this project to not only revise core concepts but also to preemptively tackle the more intricate factors often simplified in coursework for efficiency. By exploring these practical complexities, I’m preparing myself to bring a well-rounded understanding to the field.

In my last post, I covered some fundamentals, like road classifications and the concept of self-explaining roads. Now, I’m moving into a deeper element of road design: Vertical Alignment. This aspect is about managing elevation changes along the centreline of a road, which plays a crucial role in ensuring driver safety and comfort.

What is Vertical Alignment?

Vertical alignment refers to the longitudinal profile of a road, comprising grades and vertical curves. It’s shaped by a mix of planning requirements, access points, topography, geology, design controls, and economic considerations. Correctly setting the vertical alignment is fundamental to creating a safe and smooth road experience.

Vertical alignment allows for smooth transitions along road grades by connecting them with vertical curves. These curves are typically parabolic and are defined by a K value. This K value represents the length (in metres) needed for a 1% change in grade and is used to determine the curve’s radius. This approach makes it easier to create gentle elevation adjustments across the road.

Grades

In general, grades should be kept as flat as possible while balancing drainage needs and cost-effectiveness. However, very steep grades can lead to significant speed differences between vehicles, which increases rear-end collision risk. Austroads (2014b) notes that casualty crashes tend to increase with steeper grades, particularly on downhill slopes, with a noticeable rise in crash frequency and severity when the grade exceeds 6%.

• Maximum Grades: Grades are largely influenced by vehicle performance, the desired level of service, and the terrain. For more challenging terrain or higher-priority roads, steeper grades may be permitted.

• Minimum Grades: Minimum grades are primarily needed to ensure proper drainage, preventing water pooling. At certain vertical curve points, such as crests or sags, grades may fall below the minimum; in these cases, crossfalls steeper than 2% are advised.

Vertical Curves

Vertical curves are essential in connecting grades smoothly and are classified into four main types:

• Crest Curve: A positive grade followed by another positive grade
• Sag Curve: A positive grade followed by a negative grade
• Crest Curve: A negative grade followed by another negative grade
• Sag Curve: A negative grade followed by a positive grade

Steps for Calculating a Vertical Curve

In my project, I learned how to calculate vertical curves through the following steps:

1. Calculate Grades g1 and g2

• Use the formula for grade calculation: Rise / Run

2. Calculate Vertical Curve Length (VCL or VCR)

• VCL = VCR X |g1-g2| / 100
• VCR = 100 X VCL / |g1-g2|

3. Calculate Tangent Point (TP) Chainages for TP1 and TP2

• CH TP1 = CH VIP2 - VCL / 2
• CH TP2 = CH VIP2 + VCL / 2

4. Calculate Tangent Point (TP) Elevations for TP1 and TP2

• EL TP = EL VIP 2 +- (VCL/2 X |Grade|/100)

5. Plot and Label Tangent Points

• Plot the tangent points onto the road's longitudinal section and label them accordingly.

Below are the calculations and diagrams I developed for this project, illustrating the process.

Real-World Considerations

eyond the formulas, real-world applications require factoring in variables like sight distance, driver reaction time, and lighting conditions. Austroads guidelines recommend adjusting the K value to reflect these conditions, as they play a vital role in real-world design.

Question – What Tools Improve Efficiency in Real-World Road Design Calculations?

While working on my school project, I manually calculated each element step-by-step. I’m curious—what solutions or tools are used in the field to streamline these kinds of calculations and improve efficiency in real-world road design? I’d love to hear your thoughts and experiences.

Reference

Austroads (2021) Guide to Road Design Part 3: Geometric Design (2021), Austroads website, accessed 1 November 2024, https://austroads.com.au/publications/road-design/agrd03/media/AGRD03-16_Guide_to_Road_Design_Part_3_Geometric_Design_Ed3.4.pdf.

Michael A and Sean Q (2020) CEA1216 Civil Design and Drafting: Student Workbook Part A: Road Design, TAFR Queesnland.