True, but the way people drive these days, the people coming from Springfield towards Marple tend to get over in the far right-hand lane to go to the new Giant, so it's really a sticky situation for those making the right onto 320. Better safe than sorry. I know that when I'm coming from Springfield, I'm afraid those who are yielding will pull over into my lane of traffic.

I feel your pain, however this really isn't a yield spot either. The right turn from 420 to 320 is into its own/new lane on 320 so the cars should not stop/pause at all! This was indicated with the new poles, however people still don't seem to realize this.
There is a similar situation when exiting 476NB onto Baltimore Pike towards the Springfield Mall, the right turn dumps off traffic into it's own, new lane, you need to merge further down the road and don't need to yield to traffic at this point.
Perhaps this needs to be taught to drivers?

Re: 320/420 - just because a new lane opens up, doesn't mean you can proceed without yielding. The new lane is also needed for the traffic already on 320 that needs to make a right turn, and thus needs to be in that lane. If you turn off 320 into that lane, you're blocking their lane change and making things dangerous. They have the right of way, and the 420 traffic needs to yield.