Lake Constance is large, and the road that goes around it is narrow and there is only one. The speed there is severely limited almost everywhere, and for those who are too fast, not noticing the signs, there are radars. There is almost always a traffic jam from may to september, because of too many people.
Public transport can help, but it is not cheap. This page is about what public transport around Lake Constance is available. Read more about guest and tourist cards – Tourist and guest cards of Lake Constance