I have the same problem. ~15 mile ride to work and I feel nervous in just jeans (armoured jacket & boots of course). I wore leathers in to work a few times, and just changed into jeans I carried in a backpack, but it was a hassle. Especially the times I get pulled into a meeting as soon as I walk through the door. I bought these icon field armour shin/knee guards:
http://www.amazon.com/Icon-Field-Armor-Knee-Guards/dp/B001EQN3T4
...I feel a little more protected...also the description says that it 'fits under most denim jeans'...that's pretty accurate. I have a several jeans that have narrow pant legs that makes these difficult/impossible to get on/off. My calves are fairly large, so these might work better for someone with smaller legs.
http://www.amazon.com/Icon-Field-Armor-Knee-Guards/dp/B001EQN3T4
...I feel a little more protected...also the description says that it 'fits under most denim jeans'...that's pretty accurate. I have a several jeans that have narrow pant legs that makes these difficult/impossible to get on/off. My calves are fairly large, so these might work better for someone with smaller legs.