Actual Sight Distance Determination
Vertical Alignment-based 2D Calculation Method
The 2D-calculation method only uses vertical alignment information, while the horizontal (plan view) alignment and cross-sectional data is ignored. The sight distance calculation, i.e. the determination of the target point chainage using a specified eye point, is carried out using the tangent method. Any existing intermediate sags are taken into account.
The 2D calculation method leads to sufficient accuracy for:
- Very long straight alignments (purely vertical alignment-dependent sight distances)
- Rough design checks during the preliminary route planning process
- Check of vertical design elements for selected crest and sag (vertical) curve radii
- Check on the effect of overbridges or of surrounding vegetation in forested areas
- Check on illumination distances of vehicle headlamps
Sight Cone Method
The 3D method used is fully three-dimensional. The sight lines from an eye point to all possible target points through to the final cross-section combine to form a sight cone, thus resulting in a sequence of sight cones up to the relevant sighting obstruction. The term cone should be understood in its general mathematical context, namely as a central projection of a directrix to the apex of the cone.
When using this method, the relevant projection trace is determined for successive cross-sections. The individual projection trace is the result of the intersection of a sight cone with a previously specified cross-section. As the sight cones for numerous eye points must each be intersected with each cross-section in turn, the relevant projection trace on one cross-section is the result of a comparison process which is far from trivial. The calculation accuracy depends on the cross-section density, i.e. on the number of chainages defined. The more defined chainages there are, the more accurate the result is.