5V Voltage Regulator - Bypass when 5V
Since you say you want 500mA, I suggest you use a buck switching regulator such as the AOZ1022DI that behaves like a resistor if the input is lower than regulated. It has a maximum of 0.2 ohm on resistance so the voltage drop will be 0.1V maximum plus the drop across the inductor at 500mA out and 5V in. 

That is not the only one, but it's key to pick one that uses a P-channel high-side 
switch- ones that use bootstrapped N-channel MOSFETs will not provide 100% duty cycle. This particular one is rated up to 16V input (18V absolute maximum) which might or might not be too tight for your input. 

At 14V input and 500mA, an LDO will be dissipating 4.5W, which is a lot of heat to have to get rid of. A buck regulator of the above type will dissipate maybe 0.25W, maybe 20 times better.