Typical ground rent is 15% to 20% of the net cash receipts (gross revenue minus bad debt).
You don't really have a choice to determine ad rents without doing comps of the existing signs. If you can only find a few -- that is better than none. Get prices all over the general market, not just the road in question, to make sure that you have a complete handle on values.
The only other option is to "pre-sell" the advertising space, but it's very hard to pre-sell billboards unless the location is spectacular.