Have a read of this article about optimal tyre pressures at different widths and rider weights.
http://www.bikequarterly.com/images/TireDrop.pdf
15psi is a very low pressure for a tyre. The only riders who would normally even consider such low pressures are mountain bikers with very wide tyres and even then they would be worried about snake-bite punctures.
You posted in the other thread that your bike was what looked to be a 700c wheeled hybrid. I would guess with tyres around 35-38mm wide. I would suggest that if you really are riding at 15psi, you may want to try pumping that up to 50-70psi (check the tyre side walls for suggested min/max pressures). This will make your bike far easier to ride and far less susceptible to punctures and rim damage from potholes.
For some context, most road bike tyres will be 90+ psi in use (some 140+psi), most hybrid/touring tyres would be in the 50-100psi range and mountain bike tyres around 25-45psi IME.