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Transition length is the distance that is required to transition the road from normal to full superelevation. It consists of Runout Length and Runoff Length.

Runout Length is the distance that is required to transition from normal crown to zero superelevation (flat). Runoff Length is the distance that is required to transition from zero (flat) superelevation to full superelevation. 

Source : Design of Horizontal Curves 

% of Runoff Length on Tangent is the percentage of runoff length located in the tangent, and the remaining of the percentage will be in the curve region. Therefore, if the percentage is changed, the transition length will be changed as well. 

The equation used in transition length calculation can be referred as below, or can also be referred in our Appendix provided in MiTS

w        = Road Width

e         = Percentage change in superlevation

n1bw = No of lane Adjustment Factor

          = Maximum Relative Gradient

 

n1bw can be referred as table below;

Maximum Relative Gradient depends on the speed of the design. The values can be referred to the table below:

Example

We provide you the example of a project file and benchmarked with the calculation provided in the spreadsheet.

Project File

 

Given:

Design Speed (kph) = 40

Maximum Relative Gradient (refer table above) = 0.7/100 = 0.007

Road Width (m) = 3.6

No of Lanes = 2

N1bw (refer table above) = 1

Full Superelevation (%) = 6

Normal Crown (%) = -2.5

Chainage at Full Superelevation = 124.258

% of Runoff Length on Tangent = 67

 

Calculation of Runoff Length (from zero to full superelevation)

= 3.6 x |-2.5-(0)/100| x 1/ 0.007

= 12.857 m

 

Start chainage of the transition length

Given the Percentage of the Runoff Length is 67%, therefore 67% of the runoff length is located at the tangent area and the remaining 33% is located in the curve region. 

Since 67% of the runoff length is in the tangent, therefore the new runoff length is calculated as following

= 67% x 30.857m

= 20.674 m 

 

Since the runoff length at tangent is changed, therefore the transition length also will be adjusted. 

Transition Length = Runoff Length + Runout Length 

= 20.674 m + 12.857 m

= 33.531 m

 

The transition will start at 33.531 m before the full superelevation starts. Therefore, the start chainage of the transition starts at 90.727.

Start chainage of transition = Full superelevation chainage – New transition length

= 124.258 – 33.531

= 90.727 m

Based on the manual calculation, refer to the spreadsheet attached above
Based on the superelevation report
Vertical Detailing Output; the transition chainage starts at CH90.727