Highway 21 goes from I-95 to Hunting Island State Park. Highway 170 goes from Savannah to Beaufort SC.
When you search for directions on the internet, you get the URL below which says:
Hwy 170 is well known as a very dangerous highway, so please be careful on this road. The same is true on Hwy 21 going to Hunting Island State Park. Please stick to the speed limit and drive defensively.
Please, when you visit, bring plenty of bugspray. There are no-see-ums (snadgnats) and mosquitos and bite continually, especially after a rain or in the evening.
Also, bring and use at least 30 spf sunscreen. The South Carolina sun is extreme.
If you bring sunscreen and bugspray you are sure to have a fantastic time in Beaufort.