I have an idea - you could correct GPS vehicle paths with some simple hardware and software - use a $10 OBD2 dongle to get vehicle speed and than some simple computer vision (monocular visual odometry) to correct the GPS vehicle paths that are often very erratic especially within city down towns. The vision code is few lines only (like here for example) but could bring huge gains to path accuracy. The idea is use GPS for long distance correction and otherwise rely on the visual odometry.
With the setup above you may actually be even able to collect quite precise vehicle paths and use them for precise mapping. Mobileye company does something similar with their REM (Road Experience Management) technology for autonomous driving. They also collect sparse 3D data to use as "checkpoints" in their map - well this principle is very simple and I think it cold also be done within a mobile app.
Have you been thinking about doing something like this?