A control line is an imaginary line along a design for a new road. For example, (Ref. 2133) says that
“The horizontal and vertical elements of a road are described in terms of control lines. Control lines are lines mathematically defined in the horizontal and vertical planes”.
Control lines are used to calculate values for geometric parameters such as horizontal alignment (including radius), vertical alignment, and superelevation. (Ref. 584) says
“The superelevation axis of rotation shall coincide with the horizontal alignment control line and the profile control line”,
“…. with rare exceptions, the location of the axis of rotation should remain constant throughout a project”.
Having one control line for several parameters such as radius, vertical alignment and superelevation would seem to make sense. After all, in the theory behind road design all three parameters are directly related to each other (radius affects superelevation, superelevation effects radius and so on).
The effect of changing control line location is illustrated by (Ref. 2146) which says:
“The radius for horizontal curves is measured to the horizontal control line which is typically the centerline of the alignment. For wide roadways with sharp horizontal curvature, it may be appropriate to measure the radius to the inside edge of the inner most travel lane. For example, with a 5-lane undivided highway in an urban situation, the radius at the inside edge of the inner most travel lane would be 30 feet smaller than the radius at the centerline of the alignment. This could result in a significant difference in superelevation rate selected for this scenario”.
In practice, guidelines can allow for several different control lines within one road design prjejct. For example, some sources say that control lines should change with the type of road. (Ref. 710) includes the following table for superelevation control lines, where the footnotes refers to different locations for them:
Control lines can also be used for different purposes. A brief search of some English-language documents produced the folowing list:
- Vertical alignment control line
- Horizontal alignment control line
- Tangent profile control line (Ref. 827)
- True control line
- Pegged control ine (Ref. 981)
- Superelevation axis of rotation (a control line)
The type of control line can affect its position. For example (Ref. 2132) says that
“For multi-lane divided highways with relatively narrow medians, the horizontal control should be at the centerline of the median, and a single profile grade should be located at the median edge of pavement for both travelled ways”.
There can even be several control lines for the same purpose on the same type of road. For example, (Ref. 2133) says that
“On divided highways the roadways sometimes have independent vertical control lines” (own emphasis) – which means that sometimes they don’t.
There doesn’t seem to be much strict recommendation on where control lines should go (see the use of words such as “sometimes, “should be”, “typically” in the quotations above.
It seems that, looking through some guideline documents:
- The location of control lines can vary with the type of road
- A road design can involve more than one control line
- There are several different types (uses of) control lines
- The location of control lines can vary with the type of control line
- Control line location may change within a project
- Advice on control line location can vary with different guidelines (even if they come from the same country)
If you break the location of vertical, superelevation and horizontal (radius) control lines you are perhaps breaking the theory behind road design, with the possibility that in practice one or other design parameter will be below minimum requirements.
I wonder if people have over-refined the details of control lines and also perhaps lost sight of the basic road design theory. If you are asked to check the geometric design of a road you could well begin by looking at the details of control lines.
584 – USA, “Roadway design guidelines”, Arizona DOT Roadway Engineering Group, 2012
710 – Australia, “Road planning and design manual chapter 11 – horizontal alignment”, Queensland government, 2002
827 – USA, “A policy on the geometric design of highways and streets ” AASHTO, 2001;
981 – Australia, “Road design guide”, RTA of NSW, 1991
2132 – USA, “DelDOT road design manual”, Delaware DOT, 2004
2133 – Canada, “Geometric Design Manual Part 2”, Middlesex County, Ontario, Canada (downloaded 2015)
2146 – USA, “South Dakota Road design manual chapter 5: horizontal alignment”, South Dakota DOT (downloaded September 2015)