Question: Interstate 15 runs parallel to which direction of Las Vegas Boulevard?
About the article "Las Vegas Strip", section "Boundaries".

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Historically, the casinos that were not in Downtown Las Vegas along Fremont Street were restricted to outside of the city limits on Las Vegas Boulevard.
In 1959 the Welcome to Fabulous Las Vegas sign was constructed exactly outside of the city limits.
The sign is today about south of the southernmost entrance to Mandalay Bay (the southernmost casino).
In the strictest sense, "the Strip" refers only to the stretch of Las Vegas Boulevard that is roughly between Sahara Avenue and Russell Road, a distance of .
However, the term is often used to refer not only to the road but also to the various casinos and resorts that line the road, and even to properties which are not on the road but in proximity.
Phrases such as "Strip Area", "Resort Corridor" or "Resort District" are sometimes used to indicate a larger geographical area, including properties or more away from Las Vegas Boulevard, such as the Hard Rock Hotel & Casino.
The traditional definition considers the Strip's northern terminus as the now-defunct Sahara Casino, though travel guides typically extend it to include the Stratosphere, to the north.
At one time, the southern end of the Strip was Tropicana Avenue, but continuing construction has extended this boundary to Russell Road.
Mandalay Bay is located just north of Russell Road and is the southernmost resort considered to be on the Strip.
Because of the number and size of the resorts, the Resort Corridor can be quite wide.
Interstate 15 runs roughly parallel and to the west of Las Vegas Boulevard for the entire length of the Strip.
Paradise Road runs to the east in a similar fashion, and ends at St. Louis Avenue.
The eastern side of the Strip is bounded by McCarran International Airport south of Tropicana Avenue.
North of this point, the Resort Corridor can be considered to extend as far east as Paradise Road, although some consider Koval Lane as a less inclusive boundary.
Interstate 15 is sometimes considered the western edge of the Resort Corridor from Interstate 215 to Spring Mountain Road.
North of this point, Industrial Road serves as the western edge.
The famous Welcome to Fabulous Las Vegas sign is located in the median just south of Russell Road, across from the now-demolished Klondike Hotel & Casino; another similar sign is in the median at the north end of the Strip near the intersection of East St. Louis and South Main Streets.
Newer resorts such as South Point and the M Resort are on Las Vegas Boulevard South as distant as 8 miles south of the Welcome to Las Vegas sign.
Marketing for these casinos usually states that they are on southern Las Vegas Boulevard and not "Strip" properties.
State Route 147 (SR 147) is a state highway serving the Las Vegas Valley in southern Nevada.
It is signed as Lake Mead Boulevard and runs from Interstate 15 in North Las Vegas east to the border of the Lake Mead National Recreation Area.
It is not to be confused with Lake Mead Parkway (formerly Lake Mead Drive), which also goes to Lake Mead but mainly runs in southern Las Vegas Valley and carries the designation of State Route 564.
Lake Mead Boulevard became a state highway in 1978, but the highway never had markers posted to identify it as a state highway.  
The Boulevard leaves the city of Las Vegas at Sahara Avenue and assumes its unofficial name the "Las Vegas Strip" for the next .
This portion of Las Vegas Boulevard begins a few blocks to the north at the Stratosphere (the only major Strip hotel/casino actually sited within the Las Vegas city limits) and runs past Mandalay Bay ending at Russell Road.
This is the section of the road most people are familiar with; it is home to casino megaresorts with their world famous lights, huge video signs and other attractions.
It is designated as an All-American Road.
Just past the end of "The Strip", the road passes the Welcome to Fabulous Las Vegas sign (located in the median) as it abuts the western edge of the McCarran International Airport property.
The "South Strip" is now used to describe the section of Las Vegas Boulevard between Russell Road and Blue Diamond Road (which was recently relocated to intersect at Windmill Lane).
Along this stretch, development thins out except for newer shopping malls, hotels and condominiums as the Boulevard continues to travel south, just to the east of Interstate 15. 
Motorists in California leave the mountainous Mojave Desert and begin a long descent through Ivanpah Valley and Ivanpah Dry Lake.
Interstate 15 crosses the Nevada state line at the first exit, Primm.
Once Interstate 15 leaves Primm, the route travels north through the desert with few services.
The highway then enters the Las Vegas urban area upon passing the State Route 146 (Saint Rose Parkway) interchange.
Quickly thereafter, the interstate meets Interstate 215, or the Southern Beltway Interchange, which provides access to Henderson and McCarran International Airport.
Originally, this interchange was termed "Son of Spaghetti Bowl" by the Las Vegas Review-Journal when the interchange was built.
Interstate 15 travels along the west side of the Las Vegas Strip corridor and just west of downtown Las Vegas just before its junction with Interstate 515 and U.S. Route 95 (the "Spaghetti Bowl" interchange).
Then the interstate enters North Las Vegas and continues to run roughly parallel to Las Vegas Boulevard (old US 91) as it leaves the Las Vegas urban limits near the northern junction with the Clark County 215 beltway.
Once Interstate 15 leaves North Las Vegas, the freeway travels northeast and crosses the Muddy River at Glendale and then climbs up onto the Mormon Mesa.
At Mesquite, the freeway then crosses the Arizona state line and cuts through the extreme northwestern corner of Arizona through the Virgin River Gorge before entering Utah. 
The route followed a portion of Sloan Road that ran from the western Interstate 15 right-of-way east to Las Vegas Boulevard.