Minnesota's highway system evolved gradually from Native American trails linking major waterways, later used by fur traders, to military and stage roads that served as the main overland routes until railroads crisscrossed the state at the end of the 19th century. Where additional roads were needed, townships built their own, but unskilled labor and poor planning left many miles of unconnected, low-quality roads. As the state's population grew, farmers wanted dependable roads to move crops to market, merchants sought better roads to expand trade, and in the 1890s they joined bicycling advocates in the Good Roads Movement, part of a national push for all-weather, all-season thoroughfares. The arrival of the automobile at the turn of the century and heavy truck use in WW I increased demand further, while poorly marked roads hindered motorists trying to reach distant vacation destinations. To promote tourism, local boosters mapped interstate tourist routes called blazed trails, marked them with distinctive signs, and sought to persuade state and federal governments to take over those routes. Minnesota's state highway system took its present shape in 1921, when a state constitution amendment authorized the Minnesota Highway Department to oversee the design, construction, and maintenance of all state highways, bringing better planning and safer, more dependable roads. In the 1940s and 1950s, national laws created a new system of high-speed interstate highways connecting major cities, including I-35, I-90, and I-94, which cross Minnesota border to border, and today's network combines township, municipal, county, state, and federal roads into a well-integrated transportation system.