Tourism is considered another important industry in Nepal. This industry started around 1950, as the country's political makeup changed and ended the country's isolation from the rest of the world. In 1956, air transportation was established and the Tribhuvan Highway, between Kathmandu and Raxaul (at India's border), was started. Separate organizations were created in Kathmandu to promote this activity; some of these include the Tourism Development Board, the Department of Tourism and the Civil Aviation Department. Furthermore, Nepal became a member of several international tourist associations. Establishing diplomatic relations with other nations further accentuated this activity. The hotel industry, travel agencies, training of tourist guides, and targeted publicity campaigns are the chief reasons for the remarkable growth of this industry in Nepal, and in Kathmandu in particular.

Answer this question, if possible (if impossible, reply "unanswerable"): When did construction on the Tribhuvan Highway begin?
1956