# Physarum-Inspired Bicycle Lane Network Design in a Congested Megacity
Scarcity of space and other challenges are also incurred in the case of congested and unplanned megacities; thus, such expansion might not ensure green mobility, which is a necessity, today, for sustainable development [4]. As an alternative, facilitating the comfortable employment of bicycle use is explored in the present research as a promising way to enhance mobility in a congested megacity. The bicycle is an environmentally friendly means of transport, with no carbon emission, air and sound pollution, that has health and convenience benefits over motorized vehicles. Notably, motorized vehicles contribute about 20% of CO2 emissions worldwide [5]. In order to reduce carbon emissions, the bicycle is encouraged as a part of green mobility in the European Union. Riding bicycles is also good physical exercise that is beneficial to urban life worldwide. It is notable that low physical activity (or physical inactivity) in urban areas is recognized as one of the leading health risk factors causing disease, including cardiovascular, chronic lung, and heart disease, stroke, diabetes, and cancers [6]. Bicycling is popular in well-organized cities worldwide and it offers more freedom of movement, without further constraints, in crowded and unpleasant conditions. Recently, bicycling has gained more attention as a means to avoid public transportation and thus to travel in a manner safe from communicative diseases, such as COVID-19 [7; 8; 9]. Bicycles can enhance green mobility in urban areas and also safeguard accessibility in congested cities if suitable facilities are provided for a bicycle infrastructure [5]. Therefore, an effective BLN design within existing space constraints is explored in this study to enhance green mobility in congested megacities or congested areas, especially city centers or old town areas. Obviously, designing an optimal BLN, considering the complexity of congested megacities, is challenging, and such a challenging task is dealt with in this study. Designing a BLN falls within a class of network design problems that have significant differences from typical transportation network design (TND). A road network usually has to fulfill two requirements: (i) maximize direct connections between nodes to avoid large detours; and (ii) minimize costs for road construction and maintenance [10]. In general, TND concerns the optimal selection of a project from various alternatives, satisfying associated constraints to maximize cost-benefit. Various approaches have been applied to solve TND problems in the last decade, including different heuristic and bioinspired methods [11; 12; 13]. The major issue with the existing methods is optimizing network length to minimize construction costs. In contrast, the major issue of BLN design is the selection of appropriate links for optimal routes while ensuring the feasibility of the links for riding bicycles. Generally, there are two types of roads in any megacity area: the wider main roads that connect zones and serve as the backbone for inter-city communication; and the narrower link roads that interconnect different residential blocks within a zone. Managing space for a bicycle lane on the main roads is almost impossible due to massive traffic; footpaths alongside these roads are not suitable for bicycles, in most cases, as those are either narrow or mostly occupied by pedestrians and street hawkers [14]. On the other hand, link roads have issues as well, e.g., some link roads are very narrow and are left without any connections to other routes, which must be considered in BLN design. Therefore, a suitable network optimization problem needs to be formulated to design a BLN that considers the constraints mentioned above and with the preference of not disrupting traffic on the main roads. Most importantly, the desired BLN might not always be the shortest path connecting individual zones, but nonetheless be considered suitable for bicycle use.