sentences:  Since 1991, Mexico has been building toll roads that link its major cities together.Currently, there are 6,335 km worth of toll freeways in the country, with the numbering scheme of "n"-D ("n" being the number of the road bypassed, such as 45, with toll freeway as 45-D, meaning 45 Diversion).Central America's roadway network continues, linking every major city and capital, via the Pan-American Highway, which continues through Panama, across the Panama Canal, to Yaviza, Panama.