Original Article Rationality of route layouts as a quality indicator of urban road networks Banihan Gunay a , Kadir Akgol a, * and Sebastian Raveau b a Department of Civil Engineering, Akdeniz University, Dumlupinar blv., Antalya 07058, Turkey. E-mails: banihangunay@akdeniz.edu.tr; kadirakgol@akdeniz.edu.tr b Singapore-MIT Alliance for Research and Technology (SMART), 1 Create Way 138602, Singapore. E-mail: sraveau@mit.edu *Corresponding author. Abstract A number of measuring scales have been developed for the purpose of determining the acceptability levels of travel routes from the geometry point of view for urban networks. These scales have been studied under four headings: Distance as the Crow Flies, Average Speed, Alternative Routes and Angular Deflection. Besides, in order to be able to evaluate all these scales simultaneously, a common scale has been introduced. This, therefore, will enable researchers and practitioners to compare the road networks of various cities with varying size and geometry. These scales are then applied on a number of different cities around the world, and the road networks belonging to these cities are ranked according to the rationality criteria introduced. URBAN DESIGN International advance online publication, 20 April 2016; doi:10.1057/udi.2016.8 Keywords: urban planning; route choice; road network geometry Introduction Owing to congestion and population growth, urban transport has become a growing problem worldwide. The race against time has enhanced the importance attached to the ability of the urban road network to serve. Every day, commuters are confronted with the problem of selecting the opti- mum route by considering the criteria of journey time and journey length when they intend to travel from one point to another on a road network. However, the routes taken may not always neces- sarily possess the desired journey time (or length) even with ‘no traffic’ on the network. Hence, it is possible to state that, even only from the network’s geometry point of view, cities with the same size offer different levels of route qualities to the users. For example, to reach a destination located in the South of the road network, one may have to travel, say, to the East first then to the West next before directing towards the South. The amount of this diversions from the resultant direction of destination can be a quality indicator of the road network supplied. In other words, the layout (or the shape) of the route may become important in assessing the quality of the urban road network. Mathematically speaking, with respect to the journey length (with no traffic), an ideal route between any two points is a straight line joining these two points. That is, the desired road is the one that leads to the destination point without any (angular) deflection. In the real world, this becomes almost impossible particularly with long urban routes. Nevertheless, it is always desired that the length of the route between an origin and destination be close to the distance as the crow flies that joins these two points as much as possible. When comparing two different routes, the route with a ratio of route length to its distance as the crow flies close to unity or the route yielding the smallest value of this ratio can be considered more ‘rational’, as will be discussed in detail later in the article. © 2016 Macmillan Publishers Ltd. 1357-5317 URBAN DESIGN International 1–13 www.palgrave-journals.com/udi/