What's the difference between 3 17 and 3 18?

The highway connecting Sichuan and Tibet is called Sichuan-Tibet line, which is divided into the south of Sichuan-Tibet line and the north of Sichuan-Tibet line. The southern line is what we usually call national highway 3 18, and the northern line is national highway 3 17.

Southern section of Sichuan-Tibet line:

Chengdu → 147KM→ Ya 'an → 168KM→ Luding →49KM→ Kangding →75KM→ xinduqiao →74KM→ Yajiang → 143KM→ Litang → 165KM→ Batang → 36km. 90KM→ Ranwu → 129KM→ Bomi →89KM→ Tongmai → 127KM→ Linzhi → 19KM→ Bayi → 127KM→ Gongbu Jiangda →206KM→ Mozhugongka → 68km.

The southern section of the Sichuan-Tibet line, with a total length of about 2 140km, passes through alpine valleys, Jinsha River, Lancang River, Nujiang River and Yarlung Zangbo River, the famous Nyainqentanglha Mountain and Gangdise Mountain, and the Dadu River, Yalong River, Erlang Mountain, Zheduo Mountain and Galsi Mountain in Sichuan.

Northern Sichuan-Tibet line:

Chengdu →48KM→ Dujiangyan →355km→ Danba → 160KM→ Dawu →72KM→ Luhuo →97KM→ Ganzi → 106KM→ Manigango →12km → Dege → 24k.

Different from the southern part of Sichuan-Tibet line, the northern part of Sichuan-Tibet line can be divided into "Xiaobei line" and "Dabei line". The Xiaobei Line starts from Chengdu, directly enters Ganzi Prefecture via Ya 'an, enters Tibet via Luhuo, Ganzi and Degegang Jinsha River Bridge, then reaches Bangda, the intersection of the North and South Lines via Jiangda and Changdu, and then reaches Lhasa via Bomi and Bayi.

The Dabei Line runs north from Chengdu, forks at Wenchuan and National Highway 2 13, and reaches Lhasa from Brush Temple via Marcand and Changdu.