You should consult with an attorney on this. There are specific facts that your attorney will need to fully assess this situation. Your attorney will need to know, for example, how long that they've been using that road for ingress and egress. If they or a previous owner have been using that road openly without your consent for long enough, there might be a basis for a "prescriptive easement" which would give them the right to use your land and make necessary improvements such as the footbridge.
Please see an attorney so you can discuss the specific facts and figure out the best course of action. If you don't have an attorney, the Wisconsin Bar Association has a referral service that can connect you with an attorney that deals specifically with real estate issues. Go to http://www.wisbar.org