Power systems engineer here. Mixing above and below grade conductor on the same distribution line is not a common construction practice, at least not downstream of the first customer. There are a few reasons.
Most faults seen on overhead distribution systems are momentary (i.e. they clear without human invention, e.g. line-to-line squirrel contact). For this reason, automatic reclosers are typically used at the station breaker. Conversely, automatic reclose is not used on underground distribution systems because momentary contacts are rare and attempting to reclose on a permanent fault just causes further damage to your breaker and other distribution assets.
Additionally, the types of conductor used for underground and overhead applications are completely different. They'd have to be spliced going into and out of the duct (you don't just put wires in the ground, you have to pour a concrete-encased duct) which adds an additional point of failure.
I understand that have two outages per year is annoying, but the thing about power distribution is that making a perfect system is much, much more expensive than making one that works most of the time for most of the people. And that money comes from your electricity rates.
The "last mile" to my neighborhood is actually underground. It's the feed to the community that's on poles, and the whole community goes down when a tree falls. I know about momentary ones where the power comes back on in a few seconds, those happen a lot, I am counting the ones where they have to send a truck out. The power is then out for from 6 hrs to 2 weeks.
As for rates, the paper ran an article a few years back on how much of our rates were based on the constant repairing of the poles - it was a big chunk, though I don't recall the details. Having a 3 man crew out there for several hours with their heavy equipment costs a bundle.
The underground section needed a repair exactly once, and the power didn't even go out.
About 2/3 of the homeowners in the community have given up and bought generators. Generators run from $500 to $5000 - I'd rather pay higher rates and not need one of those things.
The "last mile" to my neighborhood is actually underground. It's the feed to the community that's on poles
Every neighborhood within a 10 mile radius of my house is set up the same way. Poles on the main streets, underground into the cul-de-sacs.
About 2/3 of the homeowners in the community have given up and bought generators.
Eight years ago I bought a $250 gas "camping" generator. It has a peak of 4,000 watts. Running from the outdoor patio outside my walk-out basement, it's good enough to run a refrigerator, keep a computer and Internet going so I can work from home and keep cell phones charged. The recent ice storms we experienced on the East Cost left us without power for 3 days and reminded me how handy even a small generator can be.
It's on some steep slopes, and cutting the trees down will destabilize the slope, besides making people mad.
They do trim the branches now and then.
I just don't see why it's so impractical to break out the Ditch Witch and cut a trench, and lay the durned cable in it. It was done for my neighborhood, and the developer wasn't known for spending excess money :-)
In the link above, it seemed like the domain expert was quite enjoying the conversation, everything was polite and friendly and the resulting information was quite educational.
Most faults seen on overhead distribution systems are momentary (i.e. they clear without human invention, e.g. line-to-line squirrel contact). For this reason, automatic reclosers are typically used at the station breaker. Conversely, automatic reclose is not used on underground distribution systems because momentary contacts are rare and attempting to reclose on a permanent fault just causes further damage to your breaker and other distribution assets.
Additionally, the types of conductor used for underground and overhead applications are completely different. They'd have to be spliced going into and out of the duct (you don't just put wires in the ground, you have to pour a concrete-encased duct) which adds an additional point of failure.
I understand that have two outages per year is annoying, but the thing about power distribution is that making a perfect system is much, much more expensive than making one that works most of the time for most of the people. And that money comes from your electricity rates.