Research on the Impact of Expressway on the County Economy Based on a Spatial DID Model: The Case of Three Provinces of China

Table 1

Statistics of connected highway counties for 2004–2017.

Year	Guangdong	Hubei	Qinghai
Sample counties	78	67	39
2004	31	17	8
2005	37	22	8
2006	40	23	9
2007	42	27	9
2008	42	30	9
2009	44	41	9
2010	54	45	9
2011	54	50	9
2012	54	51	10
2013	58	51	10
2014	59	62	10
2015	68	62	11
2016	70	64	13
2017	73	65	22