Intervals are awesome. I have to admit that it's nearly impossible for me to do them regularly unless I have a target race of some sort to keep the motivation, but they give clear benefits in your high-end speed. (Not so much in your uber-long distance endurance.)

You should seriously consider doing intervals on your bike trainer. Yes, it sounds horrible, but you can hammer intervals until you're nearly blacking out because you don't have to worry about cars, or even staying upright. A lot of pro coaches recommend their cyclists to do their intervals on a trainer because of these reasons, even if the weather is perfect outdoors. I've found that regular indoor sessions are really useful, particularly if you have a bike computer and HRM, since you can also get controlled data on your conditioning, which is hard to do when you're on the street dealing with so many variables.