Something else to keep in mind while planning a visit to the Black Hills and Mt.Rushmore (other than The Sturgis Motorcycle Rally).
The city is getting ready to start a major rebuilding project on Mt. Rushmore Rd. (US Hwy 16). This is the road you will end up on if you take Exit 57 off of I-90 and follow the I-190 spur into Rapid City. It is the main gateway to Mt. Rushmore. It should start sometime soon and continue, at least, to 2016. The lowest bid on the 1st phase (utility relocation) was 3 million over estimate so it has been delayed for a few weeks.
Take Exit 61 and follow Elk Vale Rd. around to the south. It will connect with Hwy 79 (one way to get to Custer State Park) and will continue as Catron Blvd and connect with Hwy 16. It's mostly 4 lane with just a few stoplights.