I just tried it and it's working on my 4s. Try restarting you phone and if that doesn't work, delete and reinstall the app.
Not sure what I did wrong, but ended up with 4 apps that would not run after installation. After I installed them (and they initially worked), I did my first backup to iTunes, so perhaps something happened then? Rebooting did not help, so I deleted and then reinstalled the 4 apps - all of them are fine now.
Haven't tried MapQuest yet on the road, but it's free so I thought I'd start there. If it's not satisfactory, I think I'll go with one of the paid versions i.e., Garmin, Tom Tom, etc.
Thanks for the suggestion to delete and reinstall. Forums are great!!
Regards.