Kentucky supplemental roads and rural secondary highways are the lesser two of the four functional classes of highways constructed and maintained by the Kentucky Transportation Cabinet, the state-level agency that constructs and maintains highways in Kentucky. The agency splits its inventory of state highway mileage into four categories: The State Primary System includes Interstate Highways, Parkways, and other long-distance highways of statewide importance that connect the state's major cities, including much of the courses of Kentucky's U.S. Highways. The State Secondary System includes highways of regional importance that connect the state's smaller urban centers, including those county seats not served by the state primary system. The Rural Secondary System includes highways of local importance, such as farm-to-market roads and urban collectors. Supplemental Roads are the set of highways not in the first three systems, including frontage roads, bypassed portions of other state highways, and rural roads that only serve their immediate area.

The same-numbered highway can comprise sections of road under different categories. This list contains descriptions of Supplemental Roads and highways in the Rural Secondary System numbered 400 to 499 that do not have portions within the State Primary and State Secondary systems.

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KY 400

Kentucky Route 400 is a supplemental road in the city of Oak Grove in southern Christian County. The highway begins at US 41 Alt. (Fort Campbell Boulevard) at the eastern edge of Fort Campbell just north of the Tennessee state line. KY 400 heads east along State Line Road, which runs immediately north of the state line. The highway turns north away from the state line and veers east again to its terminus at KY 115 (Pembroke–Oak Grove Road). The Kentucky Transportation Cabinet established KY 400 through a March 30, 1987, official order. The route was added as a rural secondary highway; however, the highway was reclassified as a supplemental road at an unknown date, returned to the rural secondary system through an October 26, 2004, official order, and returned to the supplemental road system after a December 9, 2010, official order.

KY 403

Kentucky Route 403 is a route. KY 403 extends from US 231 near Woodbury north to the end of state maintenance near the Green River. KY 403 begins at US 231 (Bowling Green Road) south of Morgantown and southwest of Woodbury. The highway heads northeast as Woodbury Loop, which crosses over l-165 on its way to the city of Woodbury. KY 403 enters town from the west, turns north three blocks, and turns west and exits the city to the northwest. The highway crosses the Black Swamp Branch of the Green River and Renfrow Creek on its way to the city of Morgantown. Until March 1, 2021, there was a portion in Ohio County that was a supplemental road that ran from the Green River north to US 231 near Cromwell.

KY 403 enters the city on Porter Street. The route then turns north onto Main Street, which carries US 231 and KY 79. At G L Smith Street, the three concurrent routes turns west while Main Street continues as KY 2161. At the west end of downtown, the three highways have an oblique intersection with KY 2162, which heads east as Ohio Street and west as Logansport Road. At G L Smith Street's three-legged intersection with Veterans Way, KY 403 turns south onto Veterans Way to run concurrently with KY 70 for one block; KY 70 joins US 231 and KY 79 on G L Smith Street. One block to the south at the west end of KY 2162, KY 403 turns west onto Logansport Road. On its way to Logansport, the highway crosses I-165 and meets the east end of KY 269 (Reeds Ferry Road). KY 403 passes through Logansport, and the route ends at the end of state maintenance within a bend of the Green River.

KY 405

Kentucky Route 405 is a rural secondary highway in northeastern Daviess County. The highway heads northeast from KY 144 at Thruston. KY 405 crosses Van Buren Creek and intersects KY 1831 (Wrights Landing Road). The highway crosses Pup Creek and crosses Carpenter Lake. On either side of the lake, KY 405 intersects the loop road KY 2116. North of the lake at its junction with KY 662 at Yelvington, the highway turns northwest toward Maceo, where the route intersects CSX's LH&StL Subdivision rail line and ends at KY 2830.

KY 406

Kentucky Route 406 is a rural secondary highway in western Leslie County. The highway begins at KY 66 at the Clay County line where Upper Jacks Creek empties into the Red Bird River. KY 406 heads east along Upper Jacks Creek through Roark. East of Roark, the highway turns north out of the Upper Jacks Creek valley and then descends into the valley of Bowen Creek. KY 406 leaves that creek valley to ascend to Essie and then descends into the valley of Stinnett Creek. The highway follows Stinnett Creek east to the route's eastern terminus at US 421 next to Stinnett Creek's confluence with the Middle Fork Kentucky River at Stinnett.

KY 407

Kentucky Route 407 is a rural secondary highway in northern Christian County. The highway is a loop from US 41 (Madisonville Road) named Mannington Loop that runs from Empire north to Mannington. At Mannington, KY 407 meets the southern end of KY 1687 (Daniel Boone Road) and intersects a CSX rail line at grade just west of its terminus at US 41 just south of the Hopkins County line.

KY 408

Kentucky Route 408 is a highway in the Jackson Purchase that is split into two parts. The western segment is a rural secondary highway that extends from US 62 near Magee Springs in Carlisle County east to Hopewell Road near Viola in Graves County. The eastern segment, which contains supplemental road and rural secondary sections, extends from KY 534 near Kaler in Graves County east to US 68 near Fairdealing via the Marshall County seat of Benton.

The western piece of KY 408 begins at US 62 south of Magee Springs. The highway curves east at its junction with KY 1371 and intersects KY 307 north of Kirbyton. KY 408 briefly runs concurrently with KY 121, meets the southern end of KY 545, and crosses Lick Creek before entering Graves County. The highway passes through Wheel and briefly runs concurrently with KY 339 between a bridge over Goose Creek and a grade crossing of a Canadian National rail line. KY 408 intersects KY 440 west of Pottsville and KY 945 within the hamlet. The highway intersects US 45 and KY 1241 in West Viola. The western segment ofKY 408 intersects a Paducah & Louisville Railway line at Viola and crosses Mayfield Creek before reaching its eastern terminus at Hopewell Road.

The eastern segment of KY 408 has a joint terminus with KY 534 east of Kaler Bottoms Wildlife Management Area and southeast of Kaler. The highway enters Marshall County south of Oak Level near which the route meets the northern end of KY 2603 (Vanzora Road) and briefly runs concurrently with KY 1949 (Wadesboro Road). KY 408 continues east on Oak Level Road, which crosses Middle Fork Creek, a tributary of Clarks River. The highway curves north and then east during its concurrency with KY 2606 (Jackson School Road). KY 408 enters the city of Benton, in the western part of which the highway crosses Bee Creek, meets the southern end of KY 1558 (Ivey Road), and crosses over Purchase Parkway. The highway enters downtown Benton along 8th Street and joins US 641 and KY 58 in a concurrency along a one-way pair, Main Street southbound and Poplar Street northbound. At 12th Street, KY 58 heads west and KY 408 heads east out of downtown. KY 408 crosses Town Creek and meets the southern end of KY 1462 (Benton–Birmingham Road) before leaving the city. The highway crosses Johns River and Clarks River and meets the northern end of KY 1897 (Olive Hamlet Road) before reaching its eastern terminus on a tangent with US 68 west of Fairdealing.

KY 409

Kentucky Route 409 is a rural secondary highway in northeastern Elliott County. The highway begins at KY 486 at Stephens within a bend of the Little Fork of the Little Sandy River. KY 409 heads west along Bruin Road, which follows Ison Creek upstream and crosses Johnson Creek. The highway leaves the valley of Ison Creek and curves north and descends into the valley of Bruin Creek. KY 409 follows Bruin Creek to its northern terminus at KY 7 south of Bruin.

KY 410

Kentucky Route 410 is a supplemental road in the city of Greenup in northeastern Greenup County. The highway begins at an intersection with US 23 (Seaton Avenue) at the east end of the city. KY 410 heads north along Main Street, which intersects CSX's Russell Subdivision, then curves northwest. The highway's terminus is an arbitrary point east of KY 2541's bridge across the railroad and Main Street.

History KY 410 is a later route; the original route of KY 410 is unknown.

KY 411

Kentucky Route 411 is a rural secondary highway in eastern Butler County and southern Grayson County. The highway begins at KY 70 (Brownsville Road) at Whittinghill. KY 411 heads north along Love Lee Road, which crosses John Woolsey Creek and passes to the south and east of Love. The highway passes through Lee and Decker before entering Grayson County. KY 411 curves east and briefly runs concurrently with KY 185 (Bowling Green Road) at Ready, where the highway's name becomes Rabbit Flat Road. The highway passes through Windyville and parallels Sunfish Creek northeast to Shrewsbury. There, KY 411 turns southeast, crosses the creek, and reaches its eastern terminus at KY 187 (Shrewsbury Road).

History KY 411 is a later route; the original route of KY 411 is unknown.

KY 412

Kentucky Route 412 is a rural secondary highway in Marion County. The highway begins at KY 84 (Raywick Road) south of Saint Mary. KY 412 heads south along St. Joe Road, which follows Stewarts Creek to its confluence with Rolling Fork of the Salt River, just north of which the route meets the west end of KY 2741 (McElroy Pike). KY 412 crosses Rolling Fork and passes through Saint Joseph before curving southeast and crossing Knob Creek. The highway has a brief concurrency with KY 289 at Jessietown, during which the routes cross Slate Creek. KY 412 continues east on Jessietown Road, which crosses Moore Creek. East of Jessieville, the highway runs concurrently with US 68 and KY 55 (Campbellsville Highway). KY 412 splits northeast onto East Calvary Road and crosses Cloyd Creek immediately before its eastern terminus at KY 208 (Calvary Road) south of Calvary.

History KY 412 was formed ca. 1970.

The original KY 412 ran from US 421 north of Harlan northeast to what is now KY 2010; this became part of KY 1679 by 1969.

KY 413

Kentucky Route 413 is a supplemental road in central Harlan County. The highway begins at KY 840 in the city of Loyall. KY 840 heads south along Wilkerson Street and east on an unnamed road; KY 413 heads north along Wilkerson Street. As the highway approaches CSX's CV Subdivision, the route turns east onto County Pike and parallels the railroad east out of the city. KY 413 approaches and then runs between the Cumberland River to the south and the railroad to the north. At Baxter, the highway meets the southern end of KY 3460 (Sukey Hill Road) and has a grade crossing of the CV Subdivision at the railroad's junction with its Poor Fork Branch, which the route continues along. KY 413 meets the western end of KY 72 just north of the confluence of the Poor Fork with the Cumberland River. The highway passes under US 119 just west of its southern junction with US 421 and parallels the Poor Fork and the eponymous rail line northeast. KY 413 intersects US 421 (Pine Mountain Road) just west of the U.S. Highway's northern junction with US 119, which runs along the opposite side of Poor Fork. The highway crosses the Tom Jones Branch of Poor Fork at Gatun and has a grade crossing with the railroad before reaching its eastern terminus at an oblique intersection with KY 522 east of Rosspoint.

KY 414