Superelevation Rate Calculator Editor

Use this dialog box to edit superelevation calculated for selected curves.

Method

list the available rate calculation methods: AASHTO Methods 1 through 5, Linear with 1/R from e=0, Linear with 1/R from e=RC , and Linear with R from e=RC.

For example, consider that an automobile traveling around a curve at a given speed experiences a certain amount of centrifugal force. The above methods determine a rate of superelevation that, together with side friction, counteracts that centrifugal force.

The methods differ in that each offers a different balance in how much of the centrifugal force is opposed by friction (f) and how much is opposed by superelevation (e). The following summaries explain how much e and f are applied in each method depending on the degree of curve (DOC):

AASHTO Method 1 -- Both e and f increases in direct proportion with the degree of curve.

 

AASHTO Method 2 -- As the degree of curve increases, f is applied exclusively to counteract centrifugal force until curves require maximum acceptable friction. For sharper curves, e is applied in direct proportion to the continued increase in curvature until e reaches the maximum allowable rate.

AASHTO Method 3 -- As the degree of curve increases, e is applied exclusively to counteract centrifugal force until curves require the maximum allowable superelevation rate. For sharper curves, f is applied in direct proportion to the continued increase in curvature until f reaches the maximum acceptable value.

AASHTO Method 4 -- This method is the same as Method 3 except that it is based on the running speed and not the design speed. The running speed is typically from 83% to 100% of the design speed. As the degree of curve increases, e is applied in amounts that would counteract centrifugal force at the running speed. In addition, f is also applied in amounts such that e and f together counteract the centrifugal force of the curve at the design speed. This relationship exists up until curves requiring the maximum allowable superelevation rate. For sharper curves, f is applied in direct proportion for the continued increase in curvature until f reaches the maximum acceptable value.

AASHTO Method 5 -- Both e and f are applied increasingly through the entire range of curves that can be superelevated within the design parameters; however, the two factors do not increase linearly.

Linear with 1/R from e=0 -- Both e and f increase in direct proportion with the degree of curve. (Same as AASHTO Method 1).

Linear with 1/R from e=RC -- This method defines a specific radius (or, conversely, a degree of curvature) and a specific e value that together define a critical point on the graph of e versus DOC. Beginning at the critical point, e is applied in direct proportion to the increase in DOC, until the maximum superelevation rate is reached. For curves below the critical DOC, reverse crown is applied. However, you can override the application of reverse crown by defining the NC to RC parameter.

After you set the Rate Calculation Method to Linear with 1/R from e=RC and set the other Superelevation Parameters, the e value parameter will be activated in the RC to Super row of the Curve Limits group box. This e value, in part, defines the critical point in the graph. After you define the e value, key in a value for the Radius on the same row. This specifies that all curves of greater radius will be assigned reverse crown. If you want to override the application of reverse crown for these curves (greater than the critical radius, define the NC to RC parameter. Doing this assigns normal crown to all curves of greater radius than the NC to RC radius.

 

Linear with R from e=RC -- This method specifies a critical point in the graph of e versus DOC. The point is defined by a radius (or, conversely, a degree for curvature) and an e value. Beginning at the critical point, e is applied in direct proportion to the decrease in radius, until the maximum superelevation rate is reached. For curves below the critical radius, reverse crown is applied. However, you can override the application of reverse crown by defining the NC to RC parameter.

After you set the Rate Calculation Method to Linear with R from e=RC and set the other Superelevation Parameters, the e value parameter will be activated in the RC to Super row of the Curve Limits group box. This e value, in part, defines the critical point in the graph. After you define the e value, key in a value for the Radius on the same row. This specifies that all curves of greater radius will be assigned reverse crown. If you want to override the application of reverse crown for these curves (greater than the critical radius, define the NC to RC parameter. Doing this assigns normal crown to all curves of greater radius than the NC to RC radius.

Friction Factor

specifies the maximum coefficient of friction that can be assumed between the tires and the road at the design speed. Common values range from 0.08 to 0.20.

Design Speed

specifies the design speed (maximum safe speed) of the road in mph or kph, depending on whether Imperial or Metric is chosen for your Linear Units. The linear units parameter is set in the Tools > Options > Units and Format tab.

Running Speed

specifies the running speed, which is typically from 83% to 100% of the design speed.  This parameter is used only in  AASHTO Methods 4 and 5.

Absolute Maximum Rate

specifies the maximum allowable superelevation rate (the largest rate to assign to a curve). The AASHTO methods use this parameter to determine the sharpest curve that can be superelevated, without exceeding the design friction, for a vehicle traveling at the design speed. If the curve is sharper than the maximum, the software issues a warning that the radius is too small and that the friction factor would be exceeded.

Preferred Maximum Rate

specifies a preferred maximum superelevation rate. If this rate is exceeded for a curve, a warning message appears. This parameter is used so that you can avoid scrolling through a long list of rates.

Round Rates To (check box and field)

indicates, when checked that rounding is used.  When used, this value specifies the number of decimal places that the superelevation is rounded to.

Computed Rate

displays the computed superelevation rate based on the parameters provided.  This value is automatically recomputed when changes occur in this dialog.

Curve Limits

Use Curve Limits

specifies, when on, that superelevation is computed based on curve limits (or curve radii). The purpose of curve limits is to override theoretical superelevation rates.  For example, the calculated superelevation rate for a curve with a large radius (for example, 10,000 feet) could be less than normal crown and thus impractical for drainage.  The InRoads allows you to specify a radius above which all curves will be assigned normal crown.  Similarly, you can define a range of curves that will be assigned reverse crown.  For curves whose calculated superelevation rates are only marginally less than the maximum rate, you can apply the maximum rate.  For example, if the maximum rate is 8.0%, you could choose to apply that rate to any curve for which the calculated rate is at least 7.9%.  If your method computes 7.92%, you could choose to apply 8.0% to simplify the actual roadway construction.

The following three parameters control the curvature limits.  These define ranges that allow you to define when to maintain normal crown (NC) or reverse crown (RC), and when to apply the calculated superelevation rate or the maximum rate.

NC to RC

specifies the radius to apply normal crown to curves.

RC to Superelevation

specifies a cutoff radius such that curves of greater radius are given reverse crown, and curves of smaller radius are given the normal crown rate.  (For curves with a radius greater than NC to RC, calculated superelevation is applied.)

Start Maximum Rate

specifies the radius below which to apply the maximum superelevation rate.

Radius

specifies the curve radius.

%f used

specifies the radius at which the given percentage of lateral friction is required.

f demand

specifies the radius at which the given lateral friction is required.

e value

specifies the radius that corresponds to this e value according to the theoretical relationship between e and the radius.

Ok

executes the command.

Cancel

dismisses the dialog.

Preferences

opens the Preferences dialog.

Help

displays help.