Comparison of Hierarchical Network Design Shape Grammars for Roads and Intersections

Comparison of Hierarchical Network Design Shape Grammars for Roads and Intersections


B Vitins, N Schuessler, K W Axhausen, ETH Zurich, CH


Effects of grammars of road network design are evaluated to support future planning of efficient and sustainable networks. A new algorithm allows testing large networks for this analysis. It combines Ant Colony Optimization with a Genetic Algorithm.


Urban systems are growing fast in many countries today, and therefore face the task to build efficient transport networks. Significant productivity gains are due to infrastructure investments in urban systems. Economies benefit and rely on efficient transport systems, agglomeration processes and low trading costs. Energy consumption and emissions can be reduced considerably with appropriate network design. This research is motivated by the need of coherent methods for planning efficient, reliable and sustainable urban transport systems.

A grammar based approach is proposed to contribute to more efficient networks by reducing the enormous and complex search space in the design of a new transport network. Network grammar rules describe how streets, intersections and eventually transit lines of certain types or hierarchical levels may be added to and joined with each other; for example, if a four-lane road can be crossed by a local access road, or if a round-about can have five or more arms. Grammar rules are implicit and explicit in the relevant guidelines and norms, which regulate the design process today. There currently exist several advisory standards for transport network design in different countries, but most of them are without a fundamental research basis, remaining vague in their recommendations.

Recent developments in urban modeling and planning increasingly provide procedural methods because of their well-defined and straightforward application possibilities, allowing implementations in interactive planning tools and computer models, which go beyond current planning approaches. Additionally, grammar rules enable the practitioner to design a network within a reasonable amount of time and effort. This paper sheds light on impacts of set of different grammar rules. The impacts are estimated from a new algorithm for network generation, an area currently under-researched.

The proposed approach applies a network generation algorithm based on the integration of ant colony optimization (ACO) and a genetic algorithm (GA). For network design applications, the proposed algorithm is able to overcome restrictions of both ACO and GA, e.g. in network size, and is concurrently computationally fast. The algorithm generates best transport network layouts give a defined objective function. Different objective functions and grammar rules can be tested for their impact on the generation process of transport network layouts. The current objective function includes demand weighted travel times and travel distances as a measure of costs. An accessibility measure is implemented as an alternative for comparison reasons. A budget constraint provides an upper bound for infrastructure investment. Grammar rules are respected during the generation of network layouts. Currently examined grammar rules encompass rules on adjacent links, which determine permitted adjacent link types and the intersection types, and the number of arms allowed for each intersection type. The amount of the each land use types is predefined but alignment of the parcels in space is kept variable for optimization purposes. The resulting network layouts are evaluated with the objective functions and compared. Comparison of the outcomes provides new important evidence on the grammar rules and their impact on network design. The impacts of different grammar rules are summarized and suggestions are provided for potential guidelines.

This paper is the first comprehensive effort to account for the impacts of grammar rules, which is missing in the literature and the professional norms and guidelines. The chosen approach is independent of case studies and applicable to any urban system. Future research is proposed, accounting for additional grammar rules, for public transportation and growth processes, but also transfers to different field of network design, such as pipeline networks and telecommunication.


Association for European Transport