[Sylos]

Well, radial menus typically are displayed around your mouse cursor, so the proximity aspect is there. They also fill out the space, well, radially, so you can really just fling your cursor into a direction and will have the total width of the menu item to hit all the way.

With touch screens, there's two major differences compared to the desktop:

1) You don't have screen edges that you can fling your cursor against, so placing UI elements at the edge does not make them easier to hit.

2) Users are generally quicker to traverse the screen and hit something, but are much worse at hitting something that's small, so you often want to make UI elements bigger (which does result in them being more spaced out) and then put the UI elements on several screens instead.

[DonHopkins]

The other problem with touch screens is that your finger isn't transparent, so you can't see what you're pointing at the same as you can on a screen with a mouse. So you have to come up with different strategies for displaying menu items and feedback. Like showing the selected item title at the top of the screen where your hand isn't covering it.