The highway system of the U.S. state of New York is a network of roads owned and maintained by several jurisdictions: the state of New York through the New York State Department of Transportation (NYSDOT) and its counties, towns, villages, and cities. The most prominent of these roads are part of one of four numbered route systems in New York, each assigned at a different level of government.
Interstate Highways and U.S. Routes are assigned at the national level. Interstate Highways are numbered in a grid—even-numbered routes are east–west routes (but the lowest numbers are along Mexico and the Gulf of Mexico), and odd-numbered routes are north–south routes (with the lowest numbers along the Pacific Ocean). U.S. Routes are also numbered in a grid—even numbered for east–west routes (with the lowest numbers along Canada) and odd numbered for north–south routes (with the lowest numbers along the Atlantic Ocean). For this reason, mainline (two-digit) Interstate Highways in New York all have numbers above 78 and mainline U.S. Routes (with the exception of US 44 and US 62) all have numbers below 20. Three-digit Interstate and U.S. Highways, also known as "child routes," are branches off their main one- or two-digit "parents". The majority of Interstate and U.S. Highways are maintained by NYSDOT.
New York State Routes are assigned by NYSDOT. Most routes are completely owned and maintained by the state; however, parts of some routes are maintained by local governments, such as a county or a city. Other routes, such as New York State Route 148 in Niagara County, are completely locally owned and maintained. The shield used for state routes is a simplified version of the Seal of New York. Roads that are maintained by the state but not assigned a state route designation carry a reference route designation, which is usually posted only on small, green reference markers alongside the highway.
Two-digit routes are typically long-distance highways while three-digit routes are typically connectors between two highways of greater importance. The first two-digit routes were assigned in 1924 while the first three-digit routes were assigned as part of the 1930 state highway renumbering. At that time, three-digit routes were generally assigned in "clusters", with the Rochester area playing home to Routes 250 through 261, the North Country having Routes 177 through 195, and so on. This system was used only in the renumbering and no longer exists on a large scale due to the removal and reassignment of designations in the years since.
County routes in New York are assigned individually by each county. The numbering and signing practices vary from county to county, as does the size of each county's system. Numerical designations typically do not carry over from one county to the next; exceptions include County Route 106 in Orange and Rockland Counties, both formerly part of New York State Route 210, and County Route 60 in Chemung and Tioga Counties, both formerly part of New York State Route 17.
A brief history of the New York Roads Portal is available here.