15分钟生活圈的建成环境阈值效应研究

Research on threshold effects of built environment settings in 15-minute life-circles

15分钟生活圈的高品质空间建设需要以人为核心对建成环境进行精细优化。本文立足居民个体日常活动出行的精细化需求,提出15分钟生活圈的建成环境评估策略,即生活圈应具备日常出行吸引力,在圈内“定住”尽可能多的日常外出活动。基于此,构建基于个体的“出行定住率”指标,利用北京活动日志和建成环境数据,采用梯度提升决策树算法,探讨北京市15分钟慢行生活圈建成环境对非工作“出行定住率”的非线性影响和阈值效应,以及这种非线性影响的空间异质性。研究发现:(1) 15分钟慢行生活圈内有效的文体、医疗、零售商业设施POI点密度范围分别为3~6个/km~2、8~20个/km~2、100~300个/km~2,有效的公园绿地占比为不超过8%;(2)设施和绿地配置与非工作出行定住率的非线性关系受到空间区位的调节。最终为15分钟生活圈的建成环境精细化调整提供了科学分析的思路和方向。

In the process of social and economic transformation in China,the quality of living environment has aroused more and more attention.The 15-Minute Life-Circle Planning has received heat debates in recent years,especially in the cities of Shanghai,Jinan,Shenzhen,for the sake of meeting people’s daily needs.While an increasing number of planning guidelines for facilitating the 15-Minute Life-Circles has been issued,few studies have offered a quantitative assessment on such guidelines.The nonlinear analysis and threshold effect detection are helpful to optimize the land use of the 15-minute life-circle by providing the effective ranges of the land use variables rather than only providing the coefficient values as general linear models do.By reviewing life-circle planning of Japan and classical planning theories of Western countries,this study first proposed a measurable and operable goal of building 15-minute life-circles,that is,a successful 15-minute life-circle should capture a residents’daily trips within the area as much as possible.In other words,the goal of adjusting land uses of 15-minute life-circles is to maintain a high internal trip capture rate.Using data from a household activity survey collected in Beijing in 2017,this study then adopted the gradient boosting decision tree(GBDT)algorithm to investigate the nonlinear impact and threshold effect of the land use factors on the internal capture rate of nonwork trips in 15-minute slow-traffic life-circles.This study also investigated the spatial heterogeneity of the threshold effect of land use factors on internal capture rate.Results show that:(1)The effective ranges of the density of recreational,medical,retail and commercial facilities in 15-minute lifecircles are 3-6 POIs/km2,8-20 POIs/km2,and 100-300 POIs/km2,respectively,and the effective fraction of green space is no more than 8%;(2)The nonlinear relationships between density of facilities,fraction of green space and internal capture rate of nonwork trips are moderated by location variables.This study provides a strategy to assess the land uses of 15-minute lifecircles by applying a nonlinear method,GBDT.The findings not only provide land use specifications for planning 15-minute life-circles in Beijing,but also suggest the key points of land use adjustment in 15-minute life-circles of distinct spatial locations.