In this area, you never know when we might have cold snaps but the plants you listed are commmonly planted now (brocolli, snow peas, and lettuce). I just today direct sowed a mesclun mix and have both it and radishes planted in pots that I can bring in if we have an unusually cold snap. Covering the plants with a thick blanket will also work. I have tomato, eggplant, and pepper seedlings going but I am not even thinking about planting them outside until well into the end of April, if not the beginning of May since they love the hot weather and won't do much until the soil temps warm up significantly.

I have had broccoli in the ground since last fall. It survived our two 8oF nights uncovered, and is looking great now. I have no worries about a late freeze zapping it! I've also planted lettuce and carrots, and they'll be fine too. If you want to be careful, you can cover them and they'll definitely be fine.

We planted lettuce and radishes from seed in our raised bed in early February and they survived all the freezes (we covered them twice). They are a few weeks from being ready to eat.

So, I'd say lettuce and broccoli are just fine to plant now.

It's definitely not too early for any of the vegetables that you mentioned. I've had good luck direct seeding snow/snap peas in late Feb/early March, and, when young, the plants are very hardy. I already had peas up when we had that bad batch of weather several weeks ago (snow plus temps in the teens). I kept them covered as a precaution, and they did fine. Likewise, for a spring crop of broccoli, you definitely want to get the plants in early. I set out my transplants about two weeks ago (along with a wide variety of greens), and they've already tripled in size. Like mbuckmaster mentioned, most brassicas are hardy enough to overwinter here. As for the tomatoes and other warm weather crops, early to mid April is right on target.