Real-world proximity sensing

[kevac]

Which sensor or combination of sensors could most accurately determine proximity of objects in open space over time, and the phones are in hands, pockets or purses while walking, bicycling or driving?

How would you prioritize the accuracy of these sensors from 0-5 meters (and how accurate are they at what ranges)?
- Proximity
- Microphone/sonar/ultrasound
- Light
- Blue tooth (assuming others also had their bluetooth on as well)
- GPS (assuming data sharing with others)
- Accelerometer/gyroscope/barometer (assuming data sharing with others, post-GPS established reference point)
- IP address (assuming data sharing)