Digital Terrain Models for Road Design and Traffic Simulation

This paper presents some of requirements traffic engineers currently impose on digital terrain models from a practical point of view.The alignment of new road infrastructure must be carefully planned due to increased requirements of the Environmental Impact Analysis imposed by the European Union. Public authorities. Geometric design of roads by itself follows national standards and an engineering process including linear alignment, road cross sections and adjacent roadside environment. Calculations of the horizontal and vertical road centerline determine the three-dimensional physical location of a road considering operational, economic and environmental requirements. The geometric design of a road includes all visible features of a road notwithstanding whether it is an urban, rural or interurban road. Powerful Computer Aided Design (CAD) software enhances the ability of an engineer to conduct numerous design iterations and presentations suitable for public hearings. The digital database for project scenarios are typically based on digital terrain models (DTM) with two-dimensional grids and the perpendicular height as third dimension (z-value). The cell length of the grid and the accuracy of the z-value are relevant for the quality of the geometric design. Feasibility studies with alternative alignments require less accuracy of the height (10-20 cm) than detailed operational studies (1 cm). 3D-animation features of CAD software require the same accuracy in order to provide jerk-free movements of vehicles along the designed road. Traffic flow simulation is not only used for animation purposes but also for calculations of road capacity, fuel consumption and emissions. A method to compute energy consumption based on road alignments with height accuracy of 2-5 cm will finish up this paper. 1. GEOMETRIC DESIGN OF ROADS 1.1. Roads must fulfill different objectives imposed by traffic flow requirements, traffic safety regulations and environmental constraints. Design engineers must propose options with a suitable balance between these three objectives while taking into account the construction and maintenance cost for the public agency being responsible for the road. According to Directive 2011/92/EU of the European Parliament all projects of newly built four or more lnae roads must pass an intensive Environmental Impact Analysis (EIA). This also includes new alignments of existing roads of more than 10 km length. Less important roads may not require an EIA but still must be built according to geometrical design standards. Roads are typically classified in three main categories: motorways with grade-separated intersections, highways outside of built-up areas and urban roads. A considerable variation of national and regional design standards exist which define number of lanes, lane width, shoulder, slopes curvature, superelevation of curves, vertical curves and sight distances. The design elements are influenced by a wide variety of design controls, engineering criteria and project specific objectives (Wright and Dixon, 2004) such as  traffic safety considerations;  functional classification of the road;  type of intersections;  forcasted traffic volumes and level of truck usage;  anticipated vehicle size and weight;  topography, earth quality of the surrounding landscape and possible geotechnical treatment;  public involvement and environmental considerations. Most countries agreed upon national or regional road design standards. Some typical design standards are listed in Table 1. These standards are recommended practices for all roads, except national roads, Photogrammetric Week '13 Dieter Fritsch (Ed.) Wichmann/VDE Verlag, Belin & Offenbach, 2013