Safety Considerations

The following relationships between horizontal and vertical alignment should be applied to the design wherever possible for safety, aesthetic and drainage reasons:



A crest can obscure the horizontal alignment and the severity of a horizontal curve. Minimum radius

horizontal curves should not, therefore, be used with crest vertical curves. Lateral shifts in the alignment on crests can lead to confusion and accidents. Lateral shifts of approximately one lane width are particularly hazardous as shown on Figure 6.1.

Figure 6.1: Lateral shifts on crests (poor design practice)



Crest vertical curves should be contained within horizontal curves to enhance the appearance of the crest

by reducing the three-dimensional rate of change of direction. This also improves safety by indicating the direction of curvature before the road is obscured by the crest.



The design speed of the road in both planes must be the same. This improves driver awareness of the

speed environment.



A small movement in one dimension should not be combined with a large movement in the other.



Sharp horizontal curves should not be introduced at or near the top of a crest vertical curve. The change in alignment may be very difficult to see at night.



If the crest curve restricts the driver’s view of the start of the horizontal curve, a driver may be confused and turn incorrectly. This is particularly dangerous when sharp horizontal curves are located near the crests of vertical curves (Figure 6.2).



Sharp reverse horizontal curves are undesirable in association with a crest vertical curve. The crest can obscure the reverse alignment.



A crest vertical curve or a sharp horizontal curve should not occur at or near an intersection or rail crossing.

Figure 6.2: Alignment change behind crest (poor design practice)

For good design, the horizontal curve shall indicate the change in direction before introduction of the vertical curve in both directions of travel. That is, the horizontal curve must be longer than the vertical curve as shown on Figure 6.3.

Figure 6.3: Horizontal curve longer than vertical curve (good design practice)

Intersections should not be hidden behind crest curves as shown on Figure 6.4. Intersections should be located with care to ensure that adequate sight distance is available on each approach. Intersections located in long sag vertical curves generally provide good sight distance to the intersection area.



Carriageway narrowing, changes from divided to undivided road, traffic islands and median noses should not be located at horizontal or vertical curves unless adequate visibility is available to ensure approaching drivers are aware of what is occurring. Also designs should ensure that stopping sight distance to the road surface is provided at lane diverges and lane merges.



Hidden dips, minimum re-sheets over existing pavements or minimisation of earthworks on new

construction, which create dips in the road, may reduce overall safety. Designers should avoid creating dips in vertical alignments where possible. Typical examples are shown on Figure 6.5 and Figure 6.6, with Figure 6.7 providing guidance to improve grading. Where correction of an existing alignment is uneconomic, other cues should be provided for drivers, such as guide posts.



The situation illustrated in Figure 6.5 is most undesirable as there is a potential hazard that a driver may attempt to overtake, unaware that a vehicle is approaching in the opposite direction. Although shallow dips (Figure 6.6) are less hazardous, approaching drivers cannot know whether a dip hides an on-coming vehicle. Dips should be avoided on long uniform grades (Figure 6.7), particularly on straight alignments.



Compound curves; uni-directional curves of considerably different radii should be avoided (Section 7.5).

Figure 6.5: Hidden dip (poor design practice)

Figure 6.6: Shallow dip (poor design practice)

Figure 6.7: Measures to correct dips in long uniform grades

The vertical curve overlaps one end of the horizontal curve

If a vertical crest curve overlaps either the beginning or the end of the horizontal curve, drivers have little time to react to the horizontal curve once it comes into view. This is a particularly unsafe practice if there is a decrease in the operating speed at the start of the horizontal curve.

The defect may be corrected in both cases by completely separating the curves. If this is uneconomic, the curves must be adjusted so that they are coincident at both ends, if the horizontal curve is of short radius. If the horizontal curve is of longer radius, they need be coincident at only one end.

The vertical curve overlaps both ends of the horizontal curve

If a vertical crest curve overlaps both ends of a sharp horizontal curve, a hazard may be created because a vehicle has to undergo a sudden change of direction during passage of the vertical curve while sight distance is reduced. This creates the same problems as discussed above.

The corrective action is to make the curves coincident at one end so as to bring the crest on to the horizontal curve.

Insufficient separation between the curves

If there is insufficient separation between the ends of the horizontal and vertical curves, a false reverse curve may appear on the outside edge-line at the beginning of the horizontal curve, or on the inside edge-line at the end of the horizontal curve. Corrective action consists of increasing the separation between the curves.

Dissimilar length horizontal and vertical geometric elements

A short movement in one plane should not be placed within a large movement in the other. A particular instance where this can lead to safety problems is when a small depression in the vertical alignment results in a ‘hidden dip’. An example of a hidden dip is shown in Figure 6.5.

Corrective action consists of making both ends of horizontal and vertical curves coincident, thus producing similar length curves. An alternative treatment is to completely separate the curves.

Long flat grades

Long straights with flat grades make it difficult for drivers to judge the distance and speed of approaching vehicles leading to overtaking accidents. An approaching vehicle more than 2500 m away on a straight seems to be standing still but the same situation on a large curve provides the driver with a changing perspective allowing some judgement of speed and distance. This situation is exacerbated at night with visibility restricted to that provided by headlights.

Roller coaster grading

Long straight sections are prone to ‘roller coaster’ grading (Figure 6.9) with the added potential for hidden dips. Designers should take care that the features are not incorporated into the design by using appropriate curvature in both planes and checking lines of sight for hidden dips.

Figure 6.9: Roller coaster grading resulting in hidden dips

In document Austroads: Guide to Road Design Part 3 Geometric Design (Page 157-163)