Any point-to-point ride can be split into a number of sections - from one set of lights to the next, from one junction to the next, or even the distance between villages for those with a more rural commute - and these provide perfect way markers to complete intervals.
Why intervals? Well, intervals allow for a period of recovery between efforts and this means you can sustain the quality of the effort and, in the end, do more training at a high intensity.
For example, 40 minutes at FTP power
is very hard to maintain - you might only manage 25 to 30 minutes depending on how fresh you are. However, if you do 4 x 10 minutes then even in the last minute of the fourth effort you should be able to sustain FTP power. Therefore, by breaking down the efforts into manageable chunks you have actually been able to do more intensive training.
Because your commute is probably the ride you do more than any other, you will have a good idea of how long it takes you to get from one marker to another.
This gives you a good idea of how much your are progressing over time. If on a day with similar wind conditions it takes you five minutes to ride between traffic lights, whereas last week it took you five-and-a-half minutes, then you know you are making improvements.
To give you a rough idea of how hard you need to be going in your intervals, you need to calculate your training zones
Interval training takes place in zone four or above. Zone four efforts should be done between way markers that are 10 to 30 minutes apart, and zone five efforts between markers one and five minutes apart. If your points of reference are closer than that then go all out!
As for the recovery between efforts, when doing intervals in zones four and five, the recovery should be roughly the same length as the interval duration. For all out sprints, take five to eight minutes between efforts.