Urban design and city analysis due to transportation network: The role of green and grey infrastructure on thermal properties of roadways in the city of Sheffield, UK

Abstract Climate change driven by anthropogenic activities is leading to atmospheric warming. In tandem rapid urbanization and densification of cities is exacerbating the urban heat island effect. A phenomenon where surface and air temperatures in urban areas is higher than that of adjacent rural areas, despite these locations experienced the same weather conditions. Urban heat islands are driven by 1. an inability to dissipate solar energy through the absorbance of incoming radiation, and the slow re-radiation of infra-red energy from hard surfaces (brick, concrete, tarmac etc.) and 2. the release of heat energy from anthropogenic activities (machinery, building heating or cooling systems, computers, vehicle engines and emissions). Although urban heat islands are understood at a city and neighbourhood level, information is still limited at a more local scale. This research aimed to partially address this by providing a better understanding of thermal behaviour around roadways, in a northern temperate-climate city, i.e. Sheffield. Specifically, the research aimed to understand how roadside structure and features influenced the local thermal properties of the roadways. Empirical experiments using high replication rates were employed to determine the influence of road location, the presence of infrastructure (buildings, trees etc.), sunlight angle and vehicle flow on local microclimates. The data confirmed previous findings that green infrastructure especially trees, but also hedges and grass provided a significant local surface cooling effect (up to 4-5oC). In contrast, hard surfaces such as offices and houses promoted higher temperatures. Temperature gradients between green and grey infrastructure were noted at an intimate scale (e.g. across roadways), but also across neighbourhoods (e.g. moving from a park towards highly dense build housing). The local cooling effect of trees on pavements and roadways was clearly demonstrated using the transactional road profile methodology. Trees at either side of the road providing a more uniform cooling affect across a roadway, than trees just at one side or the other. Higher numbers of vehicles significantly enhanced road surface temperatures (roads were warmer at rush hour, compared to equivalent quieter periods). Road temperatures were influenced strongly by the amount of incoming solar radiation and sunlight angle (time of day), but in general, the middle of the road was often warmer than other parts of the road transactional profile. This may be due to it experiencing more sunlight for longer during the day, but may also be influenced by colour of material (black tarmac) and indeed, vehicle movements. Other key factors influencing road temperatures could include the ‘openness’ of the site and degree of shading from adjacent objects (as measured by the ‘sky view factor’). These results are discussed within the context of roadway design within a changing climate.