Horizontal Curve Details
Updated July 16, 2021
This article applies to:
- RoadEng Civil
- RoadEng Forestry
This Knowledge Base article is an excerpt from one of our tutorial files. The files referenced in the article, as well as the full tutorial document, are available in our Tutorial Installer.
In this exercise, we will examine the Horizontal Curve Panel in detail.
Note: Refer to Getting Started section for file install folders (<RoadEngCivil> and <Defaults and Layouts>).
- File | Open <RoadEngCivil>\Location\Align stage 4.dsnx in Location Module.
- View | Retrieve button <Defaults and Layouts>\training\training Curve H.dlt.
Your screen should look like the figure:
Figure 4: A Screen Layout with Docked Curve Panel,
Plan Window and Section Window
Radius, Design Speed and Super-elevation
The most common way to define a safe horizontal curve is by using a super-elevation table. However, you may define curves manually if you wish.
- The first curve should already be current. If it is not, navigate to the first curve.
Figure 5: The Top Part of the Horizontal Curve Panel
- Uncheck the Use Minimum Radius box and then turn off the Auto check box (Figure 15‑5). Note that you can then define the super-elevation for a given curve manually.
- The Actual Side Friction is the coefficient of friction required to keep a vehicle on the road; alternately, it is the sideways acceleration felt by the driver (as a fraction of the acceleration of gravity, the “g-force”). The smaller the better.
|Design Speed (mph)||Radius (feet)||Super-Elevation (%)||Side Friction|
Figure 6: The Top Part of the Horizontal Curve Panel
- Turn the Auto check box back ON and press the plus button beside it to open the Auto Super Elevation Options dialogue box (Figure 15‑7).
Figure 7: Auto Super Elevation Options Dialogue Box
The use of the side friction factor method is defined in the AASHTO 2001 handbook. It relies on a table of “safe” side friction factors to calculate maximum speed given radius (or minimum radius given speed) using physical principals. It is possible to use this method to calculate the values for a super-elevation table; the two methods need not give different results. The side friction factor method is discussed in the help text and will not be discussed further here.
- Press the Select Table button to open the Lookup Table dialogue box (Figure 15‑8).
Figure 8: Super Elevation Lookup Table Dialogue Box
Note: For more information on the Super Elevation Table. Check out the Super Elevation Table Facts page on the sidebar.
- Press Cancel to close the table and Cancel again to close the Auto Super Elevation Options dialogue box.
- Sometimes when your options are limited, it is best to design alignment with the smallest safe radius – you can always increase the radius later if you have room.
- Check the Use Minimum Radius box and then check the Auto button beside Radius. In this mode, you can type a desired Design Speed and the software will use the super-elevation table to find the minimum safe radius. Try a few values.
- If you type a speed outside the table (20 mph for example) you will see the error message below left. If you type a speed greater than 40 mph (45 mph for example) you will see the warning below right. You can ignore the warning and apply the curve anyway if you wish.
Figure 9: Curve Error Messages
- Ensure the Use Minimum Radius Box is checked in the Horizontal curve panel. In this mode, you can type a desired Radius and the software will use the super-elevation table to find the maximum safe speed. Again, try a few values.
Road Class Specifications
Each curve has its own Design Speed; the maximum design speed for the entire road is specified in the Road Class Specifications.
- Press the Road Class Specifications button to open the dialogue.
Figure 10: Road Class Specifications Dialogue Box
Road Class Specifications are stored with your Template Table. Each class of road will have a few typical cross section templates, a design speed and other parameters that are common from road to road. You should have a template table (TPL file) for each class of road you may design.
If you are using a super-elevation table, the parameter in the Road Class Specifications most important for horizontal curves is the Design Speed. The other parameters are mostly concerned with vertical curves, are redundant or can be accessed directly from the horizontal curve panel. Transition Length and Widening tables are accessible from the curve panel and are discussed below.
If you turn on the Simple Curves check box you disable many features to reduce complexity – this is often used for private access roads.
- Type <F1> for more information. Press Cancel button to close the Road Class Specifications dialogue box.
The Transition Length is the distance from half-crown to full super, also known as the Super Elevation Runoff; it is labeled Length of Runoff in the figure below. In a spiral curve, the Transition Length is also the spiral distance (from tangent to spiral (TS) to spiral to circle (SC) and from circle to spiral (CS) to spiral to tangent (ST).
Figure 11: Cross Fall Behavior when Entering a Curve
- Clear the Auto box beside the two Transition Length values and note that you can now manually enter a different value for BC (begin curve) and EC (end curve).
- Check the Auto box beside the two Transition Length values then press the associated plusbutton. The table shown below will be displayed.
Figure 12: Transition Length Table.
When Transition Length is automatic, you are forced to use the same length for begin and end of curve. Like the super-elevation table, this table can be imported/exported from/to an external text file. To make changes, export, edit and re-import.
- Press Cancel to close the Transition Length table.
For a circular curve, the Super Elevation Runoff may start before the curve BC and end after; similarly, at the end of the curve, the transition starts before the EC and ends after. The Transition Fraction is the amount of transition that happens outside the curve (before BC and after EC).
For example, if Transition Length = 90 feet:
- If Transition Fraction is 1.0, the Super Elevation Runoff starts 90 feet before the BC point and full super-elevation is reached at BC.
- If Transition Fraction is 0.667, a common standard, the Super Elevation Runoff starts 60 feet before BC and full super-elevation is reached 30 feet beyond BC.
In a spiral curve, the Transition Fraction is not used (the Super Elevation Runoff always happens in the spiral section).
Tangent Runout Length
Tangent Runout length is the distance from full crown to half crown (see cross fall behaviour figure above).
- Clear the Auto check box associated with Tan. Runout (len/%). Note that you can now enter a length manually.
- Set the Auto check box associated with Tan. Runout (length). Now the caption reads len./%; the automatic value displayed is the tangent runout length for each % of crown (if your crown is 2%, you multiply this number by two to get the total length). When set to automatic, the Tangent Runout happens at the same rate as the Super-Elevation Runoff.
Small radius curves require lane widening to accommodate large vehicle off tracking. The Widening fields allow you to define a different widening distance for inside and outside lanes. Note that your cross-section template must have curve widening built in for these values to have any effect.
- As with other curve parameters, you can extract widening values from a table by setting the Auto check box. If time permits, you may wish to experiment with this feature. There is a widening table called: <RoadEngCivil>\Location\WideningFeet.tbl.
- File |Close, do not save changes.