iPhone - Showing the road speed limit while driving

Solution 1:

In the latest version of iOS 11, I believe the speed limit does show up on highways (at least when you're going too fast). Another app that does this is Waze.

Solution 2:

HERE WeGo maps app (formerly Nokia Here Maps) for iOS has exactly the functionality you are looking for.

The app can be downloaded for free on the App Store (all features available, no In-App purchases).

The app features excellent and extensive maps data, with full offline support. You can easily search for points of interest and perform navigation even without a data connection (Solely with the help of GPS).

The maps data is regularly updated and you can choose to download maps on a country/province basis.

As can be seen in the screenshot, the app not only contains up-to-date speed limit data for all major highways, but also alerts by flashing the current speed indicator in red when the limit is exceeded.