After you have removed the tube to patch it (or put in a new tube), check the inside of the tire carefully for any bits of glass, staples, or thin little pieces of wire from a steel belted vehicle tire.

Some people slide a cotton ball around the inside of the tire. The glass, staple or wire will catch on the cotton ball and hold a little cotton fluff when you move the cotton ball away.

My son had a commute like yours and was getting a flat every second day. He went for belted tires and changed jobs. Now he gets just one or two a year. We think his flats came on a particularly dirty section of road shoulder.

Good Luck