27 Jul Roadway Design Criteria
The selection of design speeds for a road or highway project is dependent on several factors including traffic volume, roadway classification, number of travel lanes and environmental factors.
Once determined, the design speed provides a framework, in accordance with local standards, for the various geometric design criteria which make up the three-dimensional footprint of a planned road, including the horizontal and vertical alignments, super-elevation rates and spiral lengths.
Working within that framework, and in order to develop an optimum design, the civil engineer must consider multiple factors in order to balance mobility and safety with respect to speed on a given roadway.
Handling Specified Design Criteria Using Civil Designer Software
Civil Designer Software allows the design and analysis of complex roads, earthworks, sewer, water and stormwater networks, at all times upholding a realistic design model in which all the relevant infrastructure elements are spatially and geographically associated.
Design criteria data is completely configurable within the software and the default installation includes standard design criteria for CD 109, TRH17, AASHTO, and TD 9_93, which covers both the South African and the UK market.
Throughout the design process, the software empowers the engineer by ensuring that all project data is at hand and available for interrogation. In addition, any deviations from acceptable geometric data ranges are flagged so that corrective alterations can be made efficiently.
The graph indicates the increasing superelevations required as the horizontal alignment curvature radius of a road decreases, and the bend becomes sharper. These are defined in accordance with specified road design speeds.
Civil Designer automatically applies this data, assigning the correct superelevation rates along the entire roadway, thereby saving considerable design time.
Handling Design Criteria variation using Road Segments
Design speeds and design criteria do not only influence the geometry and extent of proposed new roadways. The introduction of new safety regulations, the impact of greater traffic density, the introduction of new vehicle types and changing environmental factors mean that it is frequently necessary to alter existing roadways to comply with new standards.
Where different design criteria apply to different sections of the same road, for example during rural to urban environment transitions, Civil Designer allows the designer to split a single road into segments. Pre-set design criteria can then be applied to the different segments of the same road to accommodate design speed changes, without the need to change the existing design model. View a video demonstrating Road Segments in Civil Designer below (2m 48s):
Assessing Design Exceptions using Rendered Verification
In some instances, environmental or other constraints make the stipulated geometric parameters impractical or too expensive and it may be necessary to design a road, or road segment, that does not meet specified design criteria. In these cases, the measures used to balance mobility and safety may rely on the discretion of the civil engineer. Visualization and design verification tools play an essential role in enabling the engineer to make appropriate design decisions and he or she may consider additional road furniture, traffic signage, roadside illumination provisions and other control measures in order to accommodate the departure. 3D rendered fly-throughs and accurate design model representations can be generated at any point in the design process, without the need for an additional additional software.
Whether specific design criteria are chosen for safety reasons, for service or capacity, or for comfort or aesthetic values, the designer needs to consider how best to handle their application in every roadway design project. Civil Designer allows the engineer to efficiently develop the optimum design by combining all the relevant project data into an intuitive interface, warning of design criteria deviations and automating the application of stipulated geometry