Ch 3. Combinations of Horizontal and Vertical Alignment

1. Ch 3. Combinations of Horizontal and Vertical Alignment • Use a few criteria to identify good and bad H- and V-alignment combinations • Avoid unfavorable H- and V-alignment combinations After this lecture, you will be able to:

2. Why combinations of H- and V- alignments are so important? • Extremely difficult and costly to correct alignment deficiencies after a highway is constructed. • Poorly designed combinations can spoil the good and aggravate the deficiencies of each (H- and V- alignment); Horizontal and vertical alignment should not be designed independently. They complement each other.

3. General Design Controls • The physical constraints or influences that act singly or in combination to determine the alignment are: • The character of roadway based on the traffic, topography, and subsurface conditions • The existing cultural development • Likely future developments, and • The location of the roadway’s terminals Design speed determines limiting values for elements of a highway (like radius, v-curve length, etc).

4. General Design Controls • Curvature and grades should be in proper balance. • Vertical curvature superimposed on horizontal curvature, or vice versa, generally results in a more pleasing facility. • Sharp horizontal curvature should not be introduced at or near the top of a pronounced crest vertical curve. • Somewhat related to the preceding guidelines, sharp horizontal curvature should not be introduced near the bottom of a steep grade approaching or near the low point of a pronounced sag vertical curve. • On two-lane roads and streets, the need for passing sections at frequent intervals and including an appreciable percentage of the length of the roadway often supersedes the general guidelines for combinations of horizontal and vertical alignment.

5. General Design Controls (continued) • Both horizontal curvature and profile should be made as flat as practical at intersections where sight distance is important. • On divided highways and streets, variation in width of median and the use of independent profiles and horizontal alignments for the separate one-way roadways are sometimes desirable. • In residential areas, the alignment should be designed to minimize nuisance to the neighborhood. • The alignment should be designed to enhance attractive scenic views of the natural and manmade environment.

6. Poor Poor Poor Poor Figure 3-46

7. Good Poor Good Good

8. Good Good Poor Good Poor

9. Good Poor

10. Visual effect caused by the combination of horizontal and vertical curves (1) Poor Poor Poor

11. Visual effect caused by the combination of horizontal and vertical curves (1a)

12. Visual effect caused by the combination of horizontal and vertical curves (2) Good

13. Visual effect caused by the combination of horizontal and vertical curves (3) Good

14. Visual effect caused by the combination of horizontal and vertical curves (4) Poor Poor Poor

15. Visual effect caused by the combination of horizontal and vertical curves (5) Poor Poor