N dimensions of highway engineering

Until recently engineers only thought of roads as two-dimensional objects. For example Robert Skinner (ref. 1828) says that:

“For more than 20 years, highway engineers have used two-dimensional, computer-aided drafting and design (CADD) systems to accelerate the design process and reduce  costs.   The  benefits  of  CADD  systems  have derived essentially from automating the conventional design process, with engineers doing more or less what they had done before, although much faster and with greater flexibility”.

In another example, Easa and Dabbour, in a very interesting paper (ref. 1643) say

Highway geometry is traditionally defined by considering horizontal and vertical alignments separately. Horizontal alignment is the plan view, while vertical alignment is the profile of the highway along its path. Horizontal alignment includes tangents and horizontal circular curves either with or without a transition spiral curve, while vertical alignment includes tangents (flat, upgrade, or downgrade) and parabolic curves.

This is engineering in 2D terms. However many engineers are beginning to view geometric design in rather more than just two dimensions.

3D design

I suspect that it was the growing number of ways in which GIS (Geographical Information Systems) is being applied that made highway engineers realise not only that the world really does exist in three dimensions, but that there are also ways of modelling and simulating the physical world in 3D – the vertical dimension z is designed for at the same time as the two planar dimensions x and y. Today most cities in Switzerland have 3D city models.

4D design

Adding time as a 4th dimension is becoming more common. A 2013 paper from the FHWA (ref. 1825) says that : “4D modeling allows stakeholders to visualize construction over the project duration to identify potential spatial/temporal conflicts in schedule.

Elsewhere, a 2007 paper (ref. 1787) said that: “One emerging technology is 4D CAD modeling, where a 3D CAD model is linked to a construction schedule”.

In both these cases, the fourth dimension is time.

5D design

The FHWA report also says that

“Adding a cost component to the process creates a 5th dimension, making a 5D model. Such 5D engineered models allow stakeholders to evaluate costs and model cash flows for each phase of construction”.

Other dimensions of design


This is a presently hidden variable which affects geometric road design. For example it can affect sight distance (mist ~ visibility, rain ~ side friction etc.)


Vehicle type

There has been a tendency to consider geometric design ofroads in terms of the car, and assume more or less that “what is good for the car is also good for everyone else”. This attitude is beginning to change, but perhaps it should be made more explicit: geometric design should be tested separately in terms of each type of vehicle which will use the road, and including for

  • Cars
  • Trucks
  • Bicycles
  • Buses
  • Pedestrians

The N dimensions of stopping sight distance (SSD)


This is the conventional way of designing for SSD


To refer again to Easa and Dabbour’s paper on horizontal radius using 3D, the authors argue that road design in a 3D environment  produces different answers to designing in a 2D environment. This is likely to be as true for SSD as for radius. The implication is that the way engineers conventionally design roads gives the wrong results and leads to unsafe roads (or that 3D design will produce safer roads).

4D (adding time)

Time can be considered in different scales,  from thousands of years to fractions of a second. In sight distance terms perhaps the most interesting scales are 24 hours and 12 months (The first because of the changes from day to night, the second because of the changes in weather and plant growth – decidious trees and growing crops obstruct visibility more in summer than winter)

5D (adding cost)

Introducing cost as a 5th dimension could also improve the quality of highway geometric design. It seems wrong to hide cost in the way that many design manuals presently do, when (for example) they set different minimum horizontal curve radii depending on the type of terrain. I believe this is simply a hidden way of prejudging costs (roads with large curves will always be more expensive in hilly or mountainous terrain than in flat terrain) – rather than demonstrating the presence of a real, terrain-based geometric variables (e.g. side friction in hilly areas is not different to side friction in flat terrain).


People are already talking about 5D engineering, and there seems to be no reason to stop there. Perhaps this is the beginning of a new way of looking at things.


1643 – Canada, Said M. Easa and Essam Dabbour, “Design radius requirements for simple horizontal curves on 3D alignments” Can. J. Civ. Eng; 2003)

1787 – USA, Adam Platt, “4D CAD for highway construction projects” Pennsylvania State University; 2007

1825 – USA, “3D, 4D and 5D engineered models for construction”, FHWA 2013

1828 – USA, Robert E. Skinner, “Highway design and construction – the innovation challenge”, The Bridge; 2008



About roadnotes

Robert Bartlett is an international consultant with over 30 years of professional experience as a highway and traffic engineer with leading companies and organisations in several countries, including Germany, China (Hong Kong), Qatar and the UK. Specialised in urban studies, transport and the use of GIS, research has included new ideas on subjects such as the study of social justice using GIS, the dimensions of vehicles, and comparative geometrics (highways and transport).
This entry was posted in general, roads and tagged , . Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s