Dianne,
There are several ways you can go about this. This might be one solution.
You will need to know your map resolution. Detrmine how many units are in
a 5 mile area.
Run the program r.buffer.
For the highway study.
First Step:
Run a mask on areas other than 3-5 degrees. GRASS comand r.mask
Second Step:
Run r.buffer and set the buffer to 50meters.
Greg Thacker
USACERL-TAS