Using Specific Safety Criteria in Vertical Alignment

Road grade, with crests and sags, has an influence on safety. In roadway design, some criteria are taken into account to ensure safety and provide comfort for the driver. When creating a vertical alignment, you can check if crests and sags meet sight distance criteria, even in night time conditions, and you can increase the lengths of sag/crest curves accordingly.

The Alignment Assistant dialog box contains controls for:

  • Three types of sight distance: stopping sight distance, passing sight distance, and headlight sight distance.
  • Drainage improvement.
  • Driver comfort in sag vertical curves due to the rate of grade change.

To respect these criteria (sag and crest conditions), design values are prevaluated by the administrator in Data Setup.

This task shows you how to:

Before you begin:
  • Define road design criteria in an .xml file, called in the RoadAlignmentDesign.xml in Data Setup. For more information, see Alignment Design Customization.
    Note: All the Criteria commands (Stopping sight distance, Passing sight distance, Drainage control and Passenger comfort) have a boolean value, true or false, indicating whether or not the command is automatically selected in the Alignment Assistant. In design time, you can clear or select the check box and, the last status remains persistent when you save your product. For example: <StoppingSightDistance value="True"/>.
  • Create a horizontal alignment and switch to the vertical alignment by clicking . To start the alignment profile, click Alignment Sketcher and create curves by clicking .

Use Geometric Design Rules

After determining safety goals in a script in Data Setup, you can check the vertical alignment geometry according to these sag and crest conditions.

The following design values are configured in the script:

  • The lane type: one or two ways, speed, design vehicle
  • Road profile: design speed, slope angle, the height of object for stopping and passing sight distance
  • Vehicle type (for example: car), design (for example: SUV), speed, the height of the driver's eyes, headlight height, light beam divergence
  • The speed of the vehicle type is taken into account for the length of the crest and sag vertical curve.
  • The curve type: arc or parabola.
Note: If a speed has been set for the current reference key point, it will compare each vehicle speed with the key point speed and take the lowest value as reference. If no speed has been defined for the current key point, it will compare the vehicle speed with the design speed of alignment and take the lowest value.

  1. To check the vertical alignment according to design rules, expand the Criteria section in the Alignment assistant dialog box and click one or several options.

    The optimization of vertical alignment is based on a combination of the checks (different types of sight distance). Each check option will be further detailed below.

  2. To check alignment occasionally, click Check or select Dynamic checks to launch it automatically after each design change.

    The check operation is performed to analyze the length of the crest and sag vertical curves. For example, you can check if they are long enough to satisfy the stopping sight distance criteria.

    You obtain:

    • A visualization result in the 3D area: Check of the passing sight distance only. The curve section is colored red in the vertical alignment. If the curves are long enough to satisfy this criteria, the areas are green.

    • Messages: A Message Reporting dialog box opens. When the design rules are not supported, an error of criteria is raised. For example, you are notified when a crest or sag does not meet the stopping distance criterion, or when the sight distance is below 50m. The short parabola is however created and the alignment is still valid.

  3. Right-click an error and select the Fix error command.
    If there is not enough space to fix the parabola length, a warning appears giving instructions to solve the problem manually.
  4. Remodel the detected areas until they meet the required safety standards.

    For example, you can set the minimum value as the new crest/sag length.

Check the Stopping Sight Distance

The sight distance is the length of the roadway ahead that is visible to the driver. The stopping sight distance is the sum of two distances: brake reaction distance (distance traversed by the vehicle from the instant the driver sees an object necessitating a stop to the instant the brakes are applied) and braking distance (distance needed to stop the vehicle from the instant the brake application begins). The stopping sight distance is taken into account in the parabola creation. You can detect existing crests that do not meet the stopping sight distance criterion, and improve the definition of vertical curves to enable the driver to see the road ahead and provide the distance needed to stop the vehicle.

  1. To check the vertical alignment according to design rules, expand the Criteria section in the Alignment assistant dialog box and click the Stopping sight distance option.
    Tip: To enable/disable the automatic selection of this option, you can initialize it in the script by setting the Stopping sight distance attribute to true or false. Afterwards you can select/clear this option in the Alignment assistant dialog box.
  2. Click Check.

    The vehicle properties are initialized from the design rule customization file, RoadAlignmentDesign.xml, and shared to the project team in Data Setup. You can check if the vehicle specifications (type, height, height of eye, speed, ...) meet the stopping sight distance criteria.

    The following values, defined in the design rule customization file, are checked:

    • The eye height of the driver: should be at a certain distance above the road surface.
    • The sight distance (50m is the minimum value)
    • The vehicle speed: Due to high speed, the braking distance needs to be increased on highways, especially in crests areas.
    • If a speed has been set for the current reference key point, the check operation will compare each vehicle speed with the key point speed and take the lowest value as reference.

    Stopping sight distance and headlight sight distance are linked together. If you deselect the Stopping sight distance criterion, it will automatically disable the Headlight sight distance option (the command name and check box will be grayed out but the check state will remain the same).

    Headlight sight distance is linked to stopping sight distance because it is used only in crest parabola length calculation in the stopping sight distance case. If an error linked to headlight sight distance is detected, you will be notified with a stopping sight distance error in crest.

    Error messages are listed in the Message Reporting explaining that the length of the curve does not meet the stopping distance criterion and longer sight distances are needed.

  3. Right-click an error and select the Fix error command to solve it.
    When stopping sight distance is not sufficient, a warning appears giving instructions to solve the problem manually.
  4. To allow brake reaction, improve the appearance of crest vertical curves by:
    • Giving the crest required length to the vertical curves. When there are crests, longer sight distances and longer vertical curves should be provided.
    • Increasing the vertical curve radius.
    • Reducing the design speed

Check the Passing Sight Distance

The passing sight distance is the distance ahead of the vehicle to pass safely a slower vehicle. The passing sight distance is taken into account in the parabola creation. You can measure the passing sight distance in crests, and adapt the design of vertical alignment. Passing sight distances are linked to sight distances.

On two-lane roads, faster vehicles which overtake slower vehicles, cross the centerline and use the opposite traffic lane. Drivers need to see the approaching vehicles, to have a sufficient sight distance to start and finish their passing manoeuvre without colliding with the opposing vehicle.

Critical distances must be respected: designers guarantee a given passing sight distance on at least x% of the alignment length, for example: 25% of 500m.

  1. To check the vertical alignment according to design rules, expand the Criteria section in the Alignment assistant dialog box and click the Passing sight distance option.
    Tip: To enable/disable the automatic selection of this option, you can initialize it in the script by setting the Passing sight distance attribute to true or false. Afterwards you can select/clear this option in the Alignment assistant dialog box.
  2. To set a tolerance, select a value in Length of passing sections and in Ratio of passing sections.
  3. Click Check.

    The following values, defined in the design rule script, are checked:

    • Lane type: design vehicle
    • The height of the object for passing sight distance: it can be edited in the Profile specification.
    • The eye height of the driver: should be at a certain distance above the road surface.
    • Length: The sight distance (50m is the minimum value)

    In the 3D area, the curve sections are colored red in the vertical alignment where a crest does not meet the passing distance criterion. Error messages are also listed in the Message Reporting explaining that:

    • Longer sight distances are needed.
    • The rate of the alignment on which sections respect passing sight distance.
    • The gradient is out of limit.
    • The ratio of the passing sections

  4. Right-click an error and select the Fix error command to solve it.

    When passing sight distance is not sufficient, a warning appears giving instructions to solve the problem manually.

  5. To allow the safe passing of a slower vehicle, improve the appearance of crest vertical curves by:
    • Giving the crest required length to the vertical curves. When there are crests, longer sight distances and longer vertical curves should be provided.
    • Increasing the horizontal curve radius.
    • Reducing the design speed.
  6. Rework the crests and turns to achieve your goal or add passing lanes if necessary.

Check the Headlight Sight Distance

At night, visibility is less than during the day and the vehicle's headlight illumination is restricted in sag curves lead to minimum stopping sight distance. With sags, the headlight beam intersects with the surface of the roadway which reduces the sight distance. With crests, the headlight beam hits the tangency point of the roadway surface, creating darkness at a short distance. Therefore, the height of the headlights and the length of vertical curves must be considered.

  1. To check the vertical alignment according to design rules, expand the Criteria section in the Alignment assistant dialog box and click the Headlight sight distance option (not selected by default).
    Tip: To enable/disable the automatic selection of this option, you can initialize it in the script by setting the Headlight sight distance attribute to true or false. Afterwards you can select/clear this option in the Alignment assistant dialog box.
  2. Click Check.

    The following values, defined in the design rule script, are checked:

    • Lane type: one way or two ways, design vehicle
    • Vehicle type: The headlight height and the light beam divergence can be set on each vehicle type.
    • The eye height of the driver: should be at a certain distance above the road surface.
    • The sight distance (50m is the minimum value)
    • Speed: Given the high velocity on highways (high-speed conditions), the headlight sight distance must be higher.
    • Stopping and passing sight distance: Headlight sight distance depends on these two values. The length of vertical curves must be long enough to allow safe stopping and passing distances.

    Error messages are listed in the Message Reporting explaining that longer sight distances are needed.

  3. Right-click an error and select the Fix error command to solve it.

    When headlight sight distance is not sufficient, a warning appears giving instructions to solve the problem manually.

  4. When necessary, rework the sags to improve the headlight sight distance by:
    • Giving the sag required length to the vertical curves. When there are sags, longer sight distances and longer vertical curves should be provided.
    • Increasing the horizontal curve radius.
    • Reducing the design speed.
    • Adding passing lanes.

Check the Drainage Control

To allow water drainage, road superelevation always has a normal slope at 2%. You can run drainage controls on existing parabolas in vertical alignment and adapt the vertical curves for drainage improvement on sags/crests and flatter grades.

On crests, it is recommended to ensure proper drainage near the hight point of the vertical curves, for example: at a point about 15m from the crest/sag, a minimum grade of 0.30% should be provided within 15m of the high point.

  1. To check the vertical alignment according to design rules, expand the Criteria section in the Alignment assistant dialog box and click the Drainage control option (not selected by default).
    Tip: To enable/disable the automatic selection of this option, you can initialize it in the script by setting the Drainage control attribute to true or false. Afterwards you can select/clear this option in the Alignment assistant dialog box.
  2. Click Check.

    The following values, defined in the design rule script, are checked:

    • Lane type: one way or two ways, width, slope. Given the high velocity on highways (high-speed conditions), the drainage control must be higher.
    • Grade: 15m on the left and on the right from the lowest point (sag) or the highest point (crest) of the parabola.

    Error messages are listed in the Message Reporting explaining that the curve is too flat and a minimum rate of grade is required within a certain distance of the level point.

    The Fix error contextual command is not available for Drainage control.

  3. Right-click an error and select the command to solve it.

    When drainage is not sufficient, a warning appears giving instructions to solve the problem manually.

  4. When necessary, modify the curve shape by:
    • Changing grades (in or out): modify the tangent at the start and end of the parabola.
    • Changing parabola length manually.

Check the Passenger Comfort

Undulating grade lines may cause a passenger discomfort. If the sag vertical curves have a short length, it has an effect on passenger comfort. These conditions must be considered in vertical alignment.

To improve the comfort of drivers, you can define the comfort range, and provide checks for defining the geometry that will be the safety zone.

In sag vertical curves, the rate of change of grade may lead to discomfort due to the gravitational and vertical centripetal forces.

  1. To check the vertical alignment according to design rules, expand the Criteria section in the Alignment assistant dialog box and click the Passenger comfort option (not selected by default).
    Tip: To enable/disable the automatic selection of this option, you can initialize it in the script by setting the Passenger comfort attribute to true or false. Afterwards you can select/clear this option in the Alignment assistant dialog box.
  2. To set a tolerance, select a value in Passenger comfort (in m_s2) as the centripetal acceleration threshold.
    The m_s2 value corresponds to distance in meter and the vertical acceleration (s2). The higher the value is, the higher the slope can be. If you decrease this value, the slope must be flatter.
  3. Click Check.

    The following values, defined in the design rule script, are checked:

    • Lane type: one way or two ways, speed
    • Vehicle type
    • Speed: Given the high velocity on highways (high-speed conditions), the sag vertical curves must be longer.

    Error messages are listed in the Message Reporting explaining that passenger comfort is not satisfactory in sag.

  4. Right-click an error and select the Fix error command to solve it.

    When driver comfort is not achieved, a warning appears giving instructions to solve the problem manually.

  5. To satisfy the passenger comfort, rework the sags by:
    • Giving the sag-required length to the vertical curves: decrease the gradient design to obtain a smooth grade line with gradual changes, being consistent with the type of road.
    • Increasing the horizontal curve radius.
    • Reducing the design speed.