Every driver must have sufficient visibility (sight distance) in front of the vehicle to be able to stop the vehicle in time in front of an obstacle on the road at the posted speed. This means that the available sight distance (ASD) in front of the driver must be greater than or equal to the stopping sight distance (SSD) required to stop the vehicle in front of the obstacle. The estimation method to determine the ASD varies from country to country because the SSD values, the position and height of the driver's eye and the obstacles are different. What they have in common is that the ASD on the road is first checked separately in the horizontal and then in the vertical direction, i.e., a 2D sight distance check. With the help of road design software, the sight distance on the newly designed roads can be checked simultaneously in the horizontal and vertical direction, i.e., a 3D sight distance check. In order to determine the advantages and disadvantages of 2D sight distance testing compared to 3D testing, different road models were designed according to Croatian regulation in Autodesk Civil 3D and Bentley OpenRoads Designer software. The test results showed that the sufficient ASD achieved by the 2D sight distance check on all road models does not guarantee sufficient ASD in the 3D sight distance check. To achieve sufficient ASD in the cut area of the road cross-section in crest vertical curves it is not enough just to widen the shoulders or verges, but it is also necessary to choose the radii of the vertical curves accordingly, usually larger than the radii that pass the 2D sight distance test. The above facts show that it is necessary to perform a 3D sight distance test for roads, as this also takes into account certain cross-sectional features that have a negative effect on the driver's field of vision.

rural roads; geometric design; Croatian regulation; available sight distance; 2D vs 3D analysis; crest vertical curves