I would've thought this was obvious!

I saw this sign the other day and spent about three minutes thinking, "DUH, does this REALLY require a traffic law?". About that time, a bus turned from the outer lane right in front of us, doing just what the sign said not to do. We almost caught it broadside. Perhaps the law IS needed. FYI: The sign indicates the fine is 100 Rupees (a little more the $2 U.S.)