Under KRS 177.020 the State Primary Road System classifies state-maintained roadways by the type of service and function they provide. 603 KAR 3:030 designates the following classes :
State Primary System: Interstates (symbolized by black lines on maps), parkways (brown lines) and other long distance, high volume intrastate routes (red lines) of statewide significance that generally link major urban areas within the state.
State Secondary System: Regionally significant routes (green lines) of shorter distance which provide mobility and access to land use activity, generally serving smaller cities and county seats within a region.
Rural Secondary System: Routes of sub-regional significance (yellow lines) which might include urban arterial streets and other collectors, often with access to land use activity such as farm-to-market routes as their main function.
Supplemental Roads: All other state-maintained routes (purple lines) such as frontage roads, cross roads and local access roads such as farm-to-market routes as their main function.