Southern Highlands is no where near the city of Las Vegas. It looks like Southern Highlands is unincorporated Clark County.
Here is the official city of Las Vegas map.
http://major.lasvegasnevada.gov/website/clvcamps/viewer.htm
you can navage/zoom in on the area.
You can see that the city of Las Vegas ends, generally, around Sahara.
If you look at this link and navagate to the bottom left hand area, it looks like south of St. Rose and east of LV Blvd is Henserson.
I hope this helps.