Sight Distance Analysis

Use this function for sight distance analysis. The following types of sight distance analysis can be performed:

• Stop sight analysis

• Passing sight analysis

• Meeting sight analysis

Sight distance analysis is to be performed on the road model to verify that the necessary sight distance requirements are fulfilled.

• Sight distance analysis is done for both traffic directions in one operation.

• The function is programmed with flexibility in mind, allowing the user to change most input parameters involved in calculations.

• For sight distance requirements, the user has the option of integrating a calculated requirement into the process or specifying this separately.

• The driver's (eye height) placement and target point can be modified to any position in the cross-section.

• It is also possible to apply overlay on roadside surfaces, to simulate what impact vegetation or snow might have on sight requirements.

• The results of the analysis are written to a text file, which will pop up after the analysis.

• Sightlines can also be viewed in the object model as part of the road task.

• Sightlines can also be drawn to the DWG files using the function Draw Sight Distance Lines.

Run the function from the following location in the Road task:

Analysis > Sight Distance Analysis

The dialog box Sight Distance Analysis will pop up.

Station

Define the section for which analysis is to be performed.

Method

Set the sight analysis method to use:

• Stop Sight - Calculate the stopping sight distance based on speed and friction or using a user-defined sight distance criteria

• Meeting Sight - Is based on user-defined criterias except for localized additions, see own section about localization.

  • Formula: 2x Stop Sight + 10m

• Passing Sight - Is based on user-defined criterias except for localized additions, see own section about localization.

Calculation Parameter

• Driver's eye height - The level of a sightline's starting point above the road surface (in meters).

• Object height - The level of a sightline's target point above the road surface (in meters).

• Calculate maximum sight distance - This option will calculate the maximum possible sight distance from every cross-section, but not exceed a result value limited by the value defined in the field Using Maximum Calculation Length.

  • It is important to note that calculation time will increase using this option, depending on what value is defined for maximum calculation length and the complexity of the road model.

  • Generally, the calculation time will increase more on-road models with a high standard of design and good visibility.

• When calculating sight distance, the function starts at the first cross-section by checking the visibility to a cross-section 30.0m ahead in the analysis direction and then moves the target point on to the next cross-section in the road model (with a station interval of 10.0m, i.e., 40.0m ahead).

• This stepwise calculation is repeated until visibility is obstructed or maximum calculation length is reached.

• The whole operation is then repeated moving the driver to the next cross-section.

• This iteration is performed for both traffic directions.

Placement

The position of the driver on the road surface is defined using these options.

In the case of multiple lanes the desired lane or centerline itself can be used for analysis.

• Moreover, within the selected lane, the surface edge or surface center can be used as a reference for the analysis.

• From the selected surface reference point one can also offset the position to the desired value.

  • Use positive values for a right side offset and negative values for a left offset (as seen in the direction of the analysis).

• Select the desired reference using the radio buttons.

  • In the case of using surfaces as a reference, select the desired surface for the forward and reverse directions from the respective pull-down menus.

• Define the offset value if required.

• If the number of lanes is not the same all along the road project, sight analysis should be done on a section basis.

Surface Overlay

An overlay of specified thickness can be applied to terrain and/or roadside surfaces, to simulate the impact of vegetation, snow, etc., on sight visibility.

• Apply overlay on terrain surfaces - Mark the check box to enable overlay on terrain surfaces and define the thickness.

• Apply overlay on road surfaces - Mark the check box to enable overlay on road surfaces.

  • Select the surface group from which overlay is to be applied.

  • The overlay will be applied to the surface group selected from the drop-down list to the terrain surface.

  • The setting applies to group surfaces on both sides of the road.

  • Beware of undesirable effects, if using surfaces to describe noise barriers, retaining walls, guard rails, etc.

• Note that thickness is applied vertically and not perpendicular to the surface.

Reset values

All changes to parameter settings are stored within the road model. Press Reset values to reset dialog box settings to default values. The default settings are stored in a separate INI-file (RoadSA.INI) on the Novapoint installation.

Calculate >>

Having defined all the parameters, press this button to start the analysis process.

• An intermediate message box will appear showing the progress.

• When complete, press OK, and the results will be shown in Notepad.

The calculation reports first column shows a listing of cross-sections in the forward direction (increasing stationing) and an equal listing in the reverse direction (decreasing stationing). Cross-sections are listed as interval defined in Settings - Stations.

• The second column shows the required sight distance, as calculated by the program or as defined by the user settings.

• The third column states if the sight distance requirement is fulfilled (OK) or not fulfilled (Not OK).

• The fourth column gives maximum sight distance if this option is marked, showing this value or <MAX> if the calculated distance exceeds the setting for limitation.

• The fifth and seventh columns show information about obstructions in the line of sight – what station it occurs in, the horizontal distance from centerline to obstruction, and the type of obstruction (surface type).

• If the remaining road section is shorter than the sight distance requirement, the obstruction type is given as <END OF ROAD>.

Close

When closing the dialog box, settings are stored to the road model, before returning to the Road Models main menu.

Sight Distance using Road Standard method

Calculation of minimum required Stop Sight distances

Calculation of required stop sight distance is based on physical rules of how far a vehicle will move before it stops.

This length is divided into three components:

• Reaction time - how far the vehicle will move before the driver and braking system start to react. This length component is dependent on the speed.

• Braking distance - with a given friction coefficient a vehicle will gradually reduce the speed to zero. Depending on the initial speed and this friction coefficient the braking distance can be calculated.

• Slope coefficient - the force of gravity will also influence the vehicle. The braking distance will increase when driving downhill and decrease when driving uphill.

Sight distance analysis formula

The theoretical background for sight distance (Ls) calculation uses this formula:

Ls = 0.278 * V * tr + V²/(254.3 * (f + s))

• V is design speed (user-defined).

• tr is reaction time (user-defined).

• f is friction coefficient (predefined, in the table below).

• s is slope (from vertical alignment).

This formula used as default for standards where no other localized formula is implemented.

Localization

For some road standards other formulas are implemented, and there is enabled some editing possiblities.

Meeting sight

• France - localized formula

• Norway (2008) - localized formula

• Norway (2013) - localized formula

• Norway (2018) - localized formula

Meeting sight

• Norway (2018) - editing rights

Passing sight

• Norway (2018) - possible to set speed which determins sight distance

Sight Distance using User Defined method

It is possible to insert a fixed sight distance that the sight analysis will use. In addition it is possible to add adjustments which considers longitudinal slope and curvature. The adjustments are rounded to the closest meter.

Longitudinal adjustments

The use of this is to adjust the sight distance to subtract a sight distance length when in inclines, and to add sight distances in declines.

With the fixed length set to the point of zero addition between incline and decline we can use this example:

• Incline has a max incline of 6%. The adjustment will be negative 20m (-20.00)

• Decline has a max decline of -6%. The adjustment will be positive 20m (20.00)

When the incline/decline of the road alignment is between 0 and maximum values, a linear approximation is used. Values are rounded to the closest meter.

Curvature adjustments

The use of this is to adjust the sight distance to subtract a sight distance length when in curves. The application is where the computation in a curve will give a shorter sight distance because of expected lower speed.

Considering a straight line to use the full fixed sight distance, from what radius (max. radius) will the subtraction start, and at what radius (minimum radius) will the subtraction stop - and apply the subtraction at this radius.

Example:

• Min. radius = 200. Adjustment = -20.

• Max. radius = 1750. Adjustment = 0 (not change this you have to change the fixed value at the top of the user-defined section)

When the road alignment has radius between the min/max radius values, a linear approximation is used for the adjustments. Values are rounded to the closest meter.