The SPECIFIC 328i E46 from 1999 has a max HWY only gas consumption of a little over 30mpg. That is assuming of course, no hills and you never hit traffic.
In the city, you can expect that amount to drop drastically as 328i will guzzle down fuel if you are lead footed. On my daily 30mi commutes to work, I average 26mpg mainly highway with traffic.
SOOO, to answer your question, no, 15-18 is not normal for normal driving, but, if you are flooring it, then, yea, of course it's going to be in that range.
To overcome this, learn your car's optimal operating range. Learn the position of the gas pedal that will give you what your looking for. Use the gauge on the cluster to determine what optimal is. It will take some time getting used to, but if you really are concerned with mpg, then that's what you'll need to do. Also check your air filter, tire pressure and oil. Small things added together could reduce your mpg.
Hope this enlightens you, though, don't forget, you are driving in LA, which, when I was there, had the most traffic I've ever experienced. So being as such, stop and go traffic will also kill your mpg.