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.

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

This task shows you how to:

 

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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.

  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.
  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.

  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.

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.

    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.

  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.
  2. To set a tolerance, select a value in Length of passing sections and in Ratio of passing sections.
  3. Click Check.

    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.
  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).
  2. Click Check.

    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.

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.

  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).
  2. Click Check.

    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.

  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).
  2. To set a tolerance, select a value in Passenger comfort (in m_s2) as the centripetal acceleration threshold.
  3. Click Check.

    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.