Uhhh - I don't wish to sound rude, but what do you think will solve your problem, that won't involve a little effort....or expense? Cities spend millions every year on noise mitigation.

What you COULD try that may not involve too much, is writing to your Congressperson/local member etc and see if they will bring some pressure to bear to construct noise mitigation barriers - I used to live in Sydney, Australia, and with the construction of newer freeways they constructed a lot of acoustic barriers - after a while, they started to appear along the old freeways too, where people had suffered the noise for decades. Either the roads people realised there was a certain inequity there, or the people began asking why they weren't afforded the same courtesy.

I actually think your friends' suggestions are good ones, although tree plantings would have to be very, very dense to reduce the noise - but visually and air quality wise, they'll help a lot. Trees are very good air cleaners. I don't know if it's too warm in San Diego, but people here in Central BC use decorative cedars a lot for privacy fencing = they grow tall and thick, and quickly.

I think the idea of a fountain's actually a really good idea - if it's a noisy one, the generation of 'white noise' will actually mask quite a lot. You might also consider wind chimes, kinetic sculptures and the like, but otherwise, there's not a lot you can do.

I suppose if regulations permit, you could build your own acoustic fence but that wouldn't come cheap, and won't be very effective unless you do it down your neighbours sides as well - not practical - or all of your neighbours do the same, so it goes some distance. Sound doesn't just travel in straight lines.

You haven't mentioned the noise in your house, so I guess your glazing and insulation's pretty good - so you're lucky in that regard.

There are such things as active noise cancellation devices, but I imagine they're not cheap. Cheaper, though, would be noise cancellation headphones, which I think you can wear just to mask the noise (they generate sound waves to counter noise, and are insulated) or you can listen to music AND cancel out noise.

I purchased a home on a main st too. I didn't realise the noise would bother me so much.

I planted arborvitae all the way around the property. We also added a pond with a waterfall.

Didn't help much.

I think you need a fence, as solid of one as you can afford. Solid things, like the house and neighbor's homes noticeably blocked noise.