Graphics Programs Reference
In-Depth Information
11.
In Panorama, scroll to the right until you can view the Minimum K For
Headlight Sight Distance column.
Note that a warning symbol appears in the row for Item 6 and that
the required K value is 26.000 (9.000), as shown in Figure 7.12.
FiGuRE 7.12
A warning symbol in Panorama
indicating a violation of the headlight-sight-distance criterion
▶
12.
Scroll to the left until you can see the K Value column. Change the K
value for Item 6 to
26 (9)
.
This removes the warn-
ing symbol and changes
the curve length to
215.128 (74.914). To
practice good design,
you'll round that value
in the next step.
13.
Change the Profile Curve Length for Item 6 to
225 (75)
. This generates
a K value of 27.193 (9.010), so the warning symbol is still suppressed.
14.
Save and close the drawing.
You can view the results of successfully completing this exercise by opening
Design Criteria File - Complete.dwg
.
What's So Special about K?
The K value is a way of expressing the abruptness of a vertical curve. If you've
ever been driving on a country road and caught air at the top of a hill or smacked
your head on the dashboard at the bottom of a hill, then you've experienced a K
value that is too low for your speed. The K value is calculated as a ratio between
the curve length and the change in grade. Longer, more subtle curves have a
higher K value, which equates to greater safety because the change in grade is
spread out over a longer distance.
Headlight sight distance is an important design parameter for all road designs. If
the peaks and valleys of the road don't allow headlights to illuminate obstacles
within an acceptable distance, then the road is considered unsafe. The accept-
able distance varies based on design speed. Obstacles need to be detected much
farther away at higher design speeds to allow the driver enough time to react.