Funding for this research was provided by:
Key Programs of Department of Transport of Shaanxi, China (15-42R)
Text and Data Mining valid from 2019-05-20
Received: 28 July 2018
Accepted: 7 May 2019
First Online: 20 May 2019
: The authors declare that they have no competing interests.
: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.