Transportation Geography and Network Science/Network grammars

Introduction

The application of network grammars in urban planning is increasing during recent years. Network grammars refer to transport network design shape grammars, are a set of recommendations for planners in the form of rules describing how different network elements such as nodes and links are joined with each other. They are often found in transport network planning handbooks and standards. Due to the complexity of network planning, transport network planners often use shape grammars as a network planning tool to solve NDP (network design problems) such that the design process can be eased, meanwhile, network optimization is achieved . By using network grammars, network efficiency can be maximized and a reliable and robust network can be accomplished.

Background

Application of shape grammars can be found in different fields of network design such as architecture, urban planning, transport planning, geometry and mathematics. The concept was first introduced in 1972 by the architects George Stiny and James Gips to present a formalism for specifications of paintings and sculptures. It was then adopted by various architects and urban planners. Later, Marshall focusses on transport network design and suggests grammar rules for patterns and street layouts. Van Nes evaluates the way transport networks are organised in hierarchical network structures and determines the main mechanisms behind. Vitins and Axhausen also elaborate on the existing model proposed by Marshall. Most of the commonly used grammars can be found in design handbooks and guidelines.

Advantages and limitations

This session outlines the advantages and limitations of shape grammar rules in transport network planning.

Advantages

• Shape grammar rules can be applied by network planners without in-depth transport network knowledge.

The expert knowledge in network design is vast and network planners might not have sufficient transportation knowledge to design a well-performed network, which makes network design a hard problem to solve. Shape grammar is an efficient tool to mitigate the effect on that. Shape grammar rules are often adopted from previous project experiences or guidelines suggested by professionals and generally are rules of thumbs. Their applications are straightforward, which can be done simply by following the regulations/guidelines.

• The computational costs for shape grammar are relatively low

Computational cost is the number of necessary resources required to perform the analysis. Shape grammars are often used in procedural modelling which can be used to create 3D images by applying algorithms. They are in contrast with bi-level network optimizations which require higher computational costs.

• Shape grammar rules can address topological characteristics

• Shape grammar rules can potentially overcome the complexity of networks.

They summarize the possible arrangements that maximize the network performances such that network planners are not required to assess all the possibilities of network and hence the network design can be eased.

• Shape grammars are able to adapt to different scenarios

As mentioned before, shape grammars are already applied in different fields. The reason why they have wide application is that they are adaptive to different scenarios and allow shape grammars in different disciplines joining together for completeness.

Limitations

• Lack of fundamental research base about the effect of different network designs

There are no consistent sets of recommendations and limit research on the effects of shape grammars on road network design .

• Shape grammar rules are unclear

Due to the complexity and vast knowledge in network design, the 'best design' is not yet clear, making the shape grammar vague and incomplete. Because grammars are not only used to apply physical shape but also represent functionalities such as priority rules or speed limits. As mentioned before, most of the rules are adopted from past project development such that these rules may be only applicable under certain context.

• Recommendations on dense urban road network design are often missing in the guidelines

Though urbanized areas are often important in network planning, there are not many recommendations available.

• Recommendations on reliability and robustness improvements are often missing in the guidelines

• Lack of methodology in the actual development and evaluation of rules

Applications of shape grammar rules

Applications of shape grammar are spread across the world. Below are some examples of network grammar.

• Hierarchical network design

The figure below demonstrates an example of shape grammar rule for hierarchical road network design. In this example, network elements can only be connected with each other if the adjacent links differ in one hierarchy at most to maintain a coherent network. A, B, C and D represent different network hierarchies. Road type A can be only connected to itself (same type) or type B (one hierarchy lower). The connections A-C and A-D are not suggested.

• Road expansion

Here is another example defining the direction, angle deviation and length of road expansion by . There are three different directions: straight, left or right. If the node has 1 valence (left), it is usually expanded in the straight direction. Nodes with 2 valences (middle), the direction can be taken as left or right randomly. If the node has 3 valences (right), only the remaining direction is taken. Valences are the number of incident major streets.

• Guidelines and handbooks

Many institutions provide guidelines and handbooks for planning purposes. These guidelines provide a basic mean of designing transport networks, in particular, road networks. - A Policy on Geometric Design of Highways and Streets by AASHT (American Association of State Highway and Transportation Officials) - Transportation Planning Handbook by ITE (Institute of Transportation Engineers)

Discussion

1. Why there are fewer recommendations for the dense road area, e.g. urbanized zones?

2. Grammars vary from place to place and also on a global scale. Why is that? Do you think it's good or bad?