Objectives  Rapid urbanization has brought high-rise buildings through cities, which may cause negative psychological oppressiveness on urban residents.The wide sky view can help creating a comfortable urban transportation experience. Aiming at the problem that how to estimate the sky visibility during urban transportation, we propose a new index: The sky view index-urban transportation(SVI-UT), including total sky view, distance of sky view, duration of sky view, unit distance sky view and unit duration sky view.

  Methods  The SVI-UT is defined by the ability of residents from different geographical locations in the city to see sky along the street in their daily lives to different destinations, which is a street-level index. The SVI-UT calculation comprises four steps: Obtaining street view panoramic photographs; calculating sky views based on these photographs; route planning using Web application programing interfaces; and indicators calculation. The downtown area of Kunming City is taken as the study area, and the locations in different directions of the central area are selected as the destinations.Different transportation modes are adopted to measure and analyze the sky visibility of residents during urban transportation.

  Results  SVI-UT indicators are related to two important factors: Destination and transportation modes. Residents traveling to the north and south of the main urban area (Kunming Animal Museum, Kunming Station) are more likely to be exposed to sky during transportation, while those traveling to the east (Kunming Hotel) are less likely to see such sky.For urban construction, the high-rise buildings distribution and building density in the eastern region should be considered. For urban transportation planning, residents who are exercising can choose destinations in the northern and southern regions to increase their exposure to a sky visual experience. For those traveling to destinations in the eastern region, driving as much as possible will provide higher exposure to sky. For individual dwellers, drivers, and travelers, in addition to the well-known travel costs, routes with higher SVI-UT values can be given priority selection to obtain a more comfortable travel experience. In addition, for travel without definite destinations, such as when running or walking for exercise, it can be used as a guide for the selection of transportation route.

  Conclusions  (1) The SVI-UT is a people-centered index and provides both a theoretical and innovative framework for studying human visual perception of sky during transportation. (2) The quantitative calculation framework of the SVI-UT is proposed from the public street view and transportation data. (3) The index can guide urban transportation planning from a new perspective.