This reassures the hypothesis that you probably won't catch the virus from a brief contact in a grocery store or similar if you keep your distance.
Infections happen within families, and spread from one family to another by friends/coworkers. The infected people should be easily able to list all exposed out by name.
An exception is of course mass indoors public events that are forbidden now. This is where a contact tracing app would probably prove to be the most useful.
Incidental contact is unlikely to matter in most cases. However, multiple people at different tables where infected at a restaurant from a single customer. So, it doesn’t take that much.
Yes Public transport is probably the best use case for contact tracing. A combination of location history and fare card data will work for buses. For trains it's a bit trickier since you want to resolve that you're in the same car, and Bluetooth would work well in this case.
Infections happen within families, and spread from one family to another by friends/coworkers. The infected people should be easily able to list all exposed out by name.
An exception is of course mass indoors public events that are forbidden now. This is where a contact tracing app would probably prove to be the most useful.