It may have been a while since...
Teresa,
I think it is possible that it has been a while since your Brother-in-Law visited the Hoover Dam area. The speed limit in the immediate vicinity of the dam is 25 mph and the reason people go slow is not because it is dangerous, but rather EVERYONE slows down to check out the views. Hoover dam is one of the wonders of the modern world.
Some of our lists are found on the http://www.roadtripamerica.com/tips/carcamping.htm page.
Always carry extra drinking water, "emergency can food", first aid gear, flares, basic tools, warm clothes, a pair of coveralls (for getting dirty), a good book, two flashlights and batteries, CB radio, cellular phone, blanket, teddy bear, ...
Commercial Trucks can't use it
With certain rare exceptions, (local-haul gravel trucks and some RV's) large trucks and RV's are prohibited from using US-93 over the Hoover Dam. Ever since 9/11 all trucks headed for I-15 must use Arizona county route 68 thru Laughlin and then north on US-95.
So it is just cars, SUV's and motorcycles on the route for most of the last couple of years.
It is visible -- but STOP!!!!!!!!!!!!
Sure, the Arch (there is only one) is visible from both Interstate highways, but I strongly urge you to take two hours and tour the visitor center. The movie they show that depicts the construction will give you goosebumps. The best part of a visit is the ride up to the observation platform. Truly a unique experience! Don't miss it.