When I lived about 7 miles from school, I rode my road bike in a couple of days a week in the good weather. I had to bring my clothes to school the day before. I keep hygeine supplies at school, but there are no showers, so I use the "portable bath" things you heat in the microwave. I can't put a rack on my road bike, so I took the bladder out of my Camelbak and could use that for my lunch, make up, a few papers, etc. I rode once with a regular backpack and couldn't move for days afterwards.
I just "remodeled" an older hard tail mountain bike for commuting. I have a light on the front, one blinky in the back and a rack with an Ortleib single pannier. I rode it Friday into town and it is heavy. I don't know if i want to ride it for my 15 mile commute. It definitely will take me more than an hour or 57 minutes as I would do on my road bike. The bike with the pannier has to weigh at least 30 lbs., but it does feel good for commuting to have the wider tires.
My commute is suburban, but there is traffic in a few spots, even though it is very early in the morning. Talk to me in a couple of weeks, when it is lighter at 5:45 AM!