I would still think that 15k is a bit soon for your type of driving. I would expect 25-30k minimum. I have noticed that the sensors used on these pads have always indicated (for me at least) pad replacement long before they actually needed replacement.
This is because there was a ridge of rotor developing in a minute area where the pad was not contacting. The ridge was however in line with the wear sensor. As the pads wear and come closer to the rotor, the sensor comes closer to the ridge until they meet. The sensor is grounded out and lights up.
I don't replace the sensors anymore.
'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)