Interstate 15 Business provides access to downtown Escondido as Centre City Parkway. It follows the former routing of U.S. Route 395.
I-15 Business begins at I-15 exit 28 near the southern city limits of Escondido. It runs parallel to I-15 north through the city. It intersects both the westbound and eastbound portions of County Route S6, which in this area is split into one-way roads (which are in turn separated by another street). This intersection provides access to the end of the Sprinter commuter train line. Shortly thereafter, I-15 Business intersects County Route S14, and then interchanges with State Route 78. The interchange is the last interchange on the 78 before its freeway status ends, and there are only ramps for a westbound entrance or eastbound exit. Continuing north, I-15 Business intersects with El Norte Parkway, a major arterial road, before reconnecting with Interstate 15 at I-15 exit 34 near the northern city limits of Escondido. Though the street itself continues northward well on into Temecula as a frontage road for Interstate 15, its designation as Business 15 ends at this interchange.
Interstate 15 Business in Parowan, Utah is the third business route along Interstate 15 in the state of Utah. It runs from exit 75 along Utah State Route 143, at first east on 200th Street, then north on South Main Street (Old US Highway 91) until UT 143 turns east onto East Center Street. BL 15 then continues north along Utah State Route 274 (North Main Street) eventually serving as the western terminus of Utah State Route 271 near Parowan Airport until finally terminating at Exit 78.
Interstate 15 Business is a Business loop of Interstate 15 in Bringham City, and is also concurrent with Business Loop 84. It runs from an interchange at Exit 362 on I-15/84 at first along U.S. Route 91 to the intersection with U.S. Route 89 and UT 13. US 91 continues east along US 89, while BL 15/84 turns left to go north along UT 13 (South Main Street). Further in town, the road serves as the west end of Utah State Route 90. At Forest Street, South Main Street becomes North Main Street. North of there, UT 13/BL-15/84 curves to the left onto Promontory Road, while North Main Street's designation is replaced by Utah State Route 38. From there the routes run beneath a bridge for North 500th Street West and then another bridge for a Union Pacific Railroad line, eventually ending at a folded-diamond interchange with I-15/84 at Exit 365, while UT 13 continues to run northwest through Corinne and points further north.
Interstate 15 Business in the vicinity of Tremonton, Utah is the last business route along Interstate 15 in the state of Utah. It runs from exit 376 in Elwood at the I-15/84 overlap, running north along Utah State Route 13 (4700th Street West). It also runs along a concurrency with BL-84. At the intersection with Utah State Route 102, BL-15/84 turns west onto East Main Street, while UT 13 continues north towards Riverside and Plymouth. At North and South Tremonton Street, UT 102/BL-15/84 changes from East Man Street to West Main Street, then BL 15 leaves the concurrency with UT 102/BL-84 at the intersection with Iowa String Road where it turns north. BL 15 runs along the Box Elder County Fairgrounds, then at the Bear River Hospital, makes one last left turn west onto West 1000th Street North until finally terminating at Exit 381 on I-15.
Interstate 15 Business has a length of 4.338 miles (6.981 km) through McCammon in central Bannock County. I-15 Bus. begins at I-15 Exit 44 along Jensen Road, then turns north and follows State Street to the city of McCammon. Just north of the city limits, the business route veers onto Center Street and follows that through the center of town. At the north end of the city, I-15 Bus. turns west and runs concurrently with US 30 to its northern end at I-15 Exit 47.
Interstate 15 Business spans 5.560 miles (8.948 km) through Pocatello in northern Bannock County. I-15 Bus. begins at I-15 Exit 67, where US 30 and US 91 also diverge from I-15. The business route and the U.S. Highways follow Fifth Avenue northwest. The routes split onto a one-way pair of Fifth Avenue northbound and Fourth Avenue southbound. The highways pass by Idaho State University and through downtown Pocatello. At the north end of downtown, I-15 Bus. and the U.S. Highways veer north and converge as Pocatello Avenue. US 30 turns west at Oak Street; the business route and US 91 continue north along Yellowstone Avenue. I-15 Bus. leaves US 91 when it turns east onto Alameda Road, which becomes Pocatello Creek Road when it veers northeast to the business route's terminus at I-15 Exit 71.
Interstate 15 Business has a length of 4.802 miles (7.728 km) through Blackfoot in central Bingham County. I-15 Bus. begins at I-15 Exit 89 near the northern end of the Fort Hall Indian Reservation and follows US 91 northeast parallel to the Union Pacific Railroad. The highways cross the Blackfoot River into the city of Blackfoot along Broadway Street. In downtown Blackfoot, the routes veer onto Main Street, then I-15 Bus. turns northwest onto a one-way pair, Bridge Street northbound and Judicial Street southbound. The streets converge as Bridge Street, which the business route follows to its end at I-15 Exit 93. The roadway continues beyond the interchange as US 26 and immediately crosses the Snake River.
Interstate 15 Business runs for 3.409 miles (5.486 km) through Idaho Falls in western Bonneville County. I-15 Bus. begins at I-15 Exit 116, which is also where US 26 diverges from the Interstate. The business route and U.S. Highway follow Sunnyside Road across the Snake River to Yellowstone Avenue; this intersection forms the northern terminus of US 91. I-15 Bus. and US 26 follow Yellowstone Avenue northeast parallel to the Union Pacific Railroad into downtown Idaho Falls. At the intersection of Yellowstone Avenue and Broadway Street, I-15 Bus. turns northwest onto Broadway Street and runs concurrently with US 20 Bus. across another bridge over the Snake River to their mutual termini at I-15 Exit 118. The roadway continues west as US 20.
Interstate 15 Business is a Business loop of Interstate 15 in Dillon. It runs from a diamond interchange at Exit 62 along Atlantic Street, Helena Street and Montana Street to a brief overlap with MT 41 that ends at a trumpet interchange at Exit 63.
The Interstate 15 Business Loop begins at Exit 62 across from a road leading to the Kiwanis International-established Cornell Park then after crossing a bridge over an Oregon Short Line Railroad line curves north along Atlantic Street, briefly crossing another bridge over Blacktail Deer Creek. Between East Chapman and East Poindexter Streets the route is joined by the former U.S. Highway 91. At Beaver County High School, BL-15 makes a left turn onto Helena Street and just before crossing the same OSR tracks it ran over earlier, makes a right turn onto Montana Street. Moving away from Downtown Dillon, Former US 91 branches off to the northwest, then across from the intersection with Ramshorn Street, the road turns west onto a wrong way concurrency with Montana Highway 41. BL-15/MT 41 climbs a bridge over former US 91 as well as the OSR line and Blacktail Deer Creek all at once, then makes a southbound curve before finally ending at a trumpet interchange at Exit 63.
Interstate 15 Business is a Business loop of Interstate 15 in the capital city of Helena. It runs west from exit 192 along U.S. Route 12 in Montana at the east end of a one-way pair along Prospect Avenue westbound and 11th Avenue eastbound. At North Montana Avenue, both US 12 and BL-15 turn north, but US 12 curves west onto East Lyndale Avenue between Boulder Avenue and Helena Avenue, and BL-15 follows along this curve, while North Montana Avenue continues north as Lewis and Clark County Road 229. At North Last Chance Gulch, BL-15 turns north-northeast while US 12 continues west along West Lyndale Avenue and BL-15 passes between Centennial Park and Memorial Park, the grounds of the stadium used by the Helena Brewers. After crossing a bridge over a former Northern Pacific Railroad line, BL-15 begins to turn northeast and then at the intersection of North Montana Avenue becoming Cedar Street running east, eventually ending at I-15 on exit 193 across from the Helena Regional Airport.
Interstate 15 Business in Great Falls, Montana links I-15, which bypasses downtown Great Falls to the west, to the center of Great Falls. As its Business Loop designation implies, I-15 Business terminates at I-15 (concurrent with U.S. Route 89 and Montana Highway 200 through western Great Falls) at each end. The southernmost 0.83 miles (1.34 km) of the route from the interchange with I-15 to Fox Farm Road is designated, but not signed, as Interstate 315. It is the second shortest interstate in the country, the shortest being Interstate 878.
Interstate 15 Business is a Business loop of Interstate 15 in Conrad. It runs along Old Highway 91 from diamond interchanges at Exit 335 and Exit 339. The north end was originally a half-diamond/trumpet interchange until it was rebuilt in the 2010s, and was given a rest area/truck weigh station east of the interchange.
Interstate 15 Business in Shelby, Montana links I-15, which bypasses downtown Shelby to the west, to the center of Shelby. The southern terminus of I-15 Business is at exit 363 on I-15; from there it follows U.S. Route 2 into the center of Shelby, then follows Oilfield Road across the railyard and through the north end of town to its northern terminus at exit 364 on I-15, just south of Shelby Airport.
Interstate 15 Business in Mesquite, Nevada, was also designated as State Route 144. It was removed as of December 31, 2005. It covered Mesquite Boulevard and Sandhill Boulevard in Mesquite. The State Route designation was applied to the portion of Mesquite Boulevard from its southern crossing of Interstate 15 to Sandhill Boulevard, then along Sandhill Boulevard to its northern crossing of Interstate 15. The route was 3.132 miles (5.040 km) long.