Due to the lack of adjustment index systems for taxi fleet sizes in China, this paper used the taxi operating datasets from Ningbo City and established a regression tree model to consider the endogenous indicators that affect taxi fleet sizes. Then, a dynamic adjustment mechanism of taxi fleet sizes was proposed by combining the exogenous and endogenous indicators. The importance of the exogenous and endogenous indicators was sorted using the Delphi method. The threshold value of each indicator was also given. The results indicated that (1) in the three-layer structure of the regression tree model, the mileage utilization had the strongest effect on the fleet size of taxis, and the F statistic was 63.73; followed by the average daily revenue of a single taxi, the average waiting time to catch a single taxi, the average operating time of a single taxi, and the revenue per 100 km. The overall accuracy of the model was found to be valid. (2) When the mileage utilization was less than 0.6179 and the average daily revenue of a single taxi was less than 798.38 Yuan, the fleet size of cruising taxis was surplus and should be reduced by 362 vehicles. (3) When the mileage utilization was more than 0.6774 and the average waiting time to catch a single taxi was more than 259.09 s, the fleet size of cruising taxis was insufficient, and we suggest an increase of 463 taxis.Sustainability 2020, 12, 2776 2 of 18 demand is more than the supply, it is difficult and inconvenient for the passengers to take a taxi for the trip. Otherwise, if the number of taxicabs is too high, negative effects, including traffic congestion, increases in passenger trip costs, and decreased taxi driver revenue, are generated.How to regulate a reasonable taxi fleet size to satisfy the passenger demand without sacrificing the income of taxi drivers is an important premise of taxi fleet size adjustment. Meanwhile, both the operational efficiency and service level also need to be guaranteed. Unfortunately, accurate threshold values of indicators for adjusting the fleet size of taxicabs do not exist. It is necessary for the local government to establish a reasonable adjustment method for taxi fleet size and to alleviate the conflicts between the cruising taxis and Didi and Uber. A reasonable dynamic adjustment of the taxi capacity scale can increase the mileage utilization, alleviate the traffic congestion, and reduce the carbon emissions of taxicabs. Sustainable taxi industry is an important guarantee for promoting the green development of urban traffic. Additionally, as intra-city express service develop rapidly in China, taxicabs also tried to provide express, document delivery, baggage service and other freight services in city distribution. In order to gain more money, the taxicabs, such as Shanghai city already have the permit to provide the light freight services. Therefore, taxicabs as a new type of freight service in city distribution, also need to be focused on the reasonable fleet size.Many scholars studied the forecasting and monitoring of taxi ...