Motivation: #
While applying the Maximum Relative Gradient in obtaining the Superelevation length of the Horizontal Curve, you may notice one thing- no matter what road width is applied to your design, the runout and runoff lengths do not change under the Superelevation Report.
This may confuse users, especially those who are not fluent with the Road Module environment in MiTS.
Is this a bug or intended behavior of the software? #
You may think of this behavior as a bug or hard-coded limitation in the software; this is actually intended, following the AASHTO “Green Book” (2018) standards.
If you refer to the standard under Tangent-to-Curve Transition,
The superelevation runoff lengths given in Table 3-16 are based on 12-ft [3.6-m] lanes. For other lane widths, the appropriate runoff length should vary in proportion to the ratio of the actual lane width to 12 ft [3.6 m]. Shorter lengths could be applied for designs with 10- and 11-ft [3.0- and 3.3-m] lanes, but considerations of consistency and practicality suggest that the runoff lengths for 12-ft [3.6-m] lanes should be used in all cases.
(Chapter 3, page 65)
L_{T}=\frac{wn_{1}e_{d}}{\Delta}\left(b_{w}\right)
Where:
L_{T} = minimum length of superelevation runoff, m
w = width of one traffic lane, m (typically 3.6m)
n_{1} = number of lanes rotated
e_{d} = design superelevation rate, %
b_{w} = adjustment factor for the number of lanes rotated
\Delta = maximum relative gradient, %
The relative formula above is based on the rate of change of the limit, and one of the important parameters is the road width.
The 3.6 m acts as a control width, maintaining the generalized width despite the actual, narrower width of the road. This ensures the transition from a normal, straight road to the curvy profiles is not underdesigned and becomes too abrupt, thereby abandoning the safety and comfort of drivers and passengers.
How can we apply the ‘real’ width to our design? #
The default or generalized width in MiTS has been set as 3.6 m following the standard discussed above. To ensure the ‘real’ width is applied, we need to update the generalized width in the software.
The generalized width can be set under (1) Project Settings or on the (2) Right-Hand-Side Properties of the Superelevation. However, these two parameters behave slightly differently, such that:
Project Settings #
Changes made to the generalized width under the Project Settings will only be applied to the new road (the road that is created after the changes have been made).
Options > Project Settings > Expand Road > Click on Road Width > Default Road Width, m > OK
Right-Hand-Side Superelevation Properties #
The changes made to the parameter on the Right-Hand-Side Properties will be applied instantly, on a per-road basis.
Road Module > Superelevation > Right-Hand-Side Properties
