yes usually unless it's says average it's the limit. But 15 megs is allot fo bandwidth.
We have a program that slows down traffic as you reach your limit so you just have the visitors wait some when you reach the peak.
better then most host oversubscribe and slow down sporadically now matter how much you use.
http://www.dwhs.com
Check their overage policy, we have people go over 3-5 megs from time to time and if it's no often no biggie.