Above ground or below ground is a question for the real estate and legal teams. BC Hydro spent a lot of money to build powerlines in Tsawassen on the right of way it had already bought decades ago. I also remember it being an impediment in Port Coquitlam for the Evergreen line where a few industrial landowners including the mayor were holding out? The actual construction cost and difference in operating cost probably won't determine the best option.
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The one thing people in Vancouver do not appreciate enough is time.
It's worth going back 50-60 years to a time when much of the current infrastructure was built, or neighbourhoods were established in their modern form. Compare that to the present and then think about what the next 50-60 years holds because that will be the next time the questions of today are asked.
Broadway of 50-60 years ago looked like central Surrey does today. The Burrard and Granville bridges were overbuilt like the Golden Ears and Port Mann are now. (fyi: Newton, Whalley, City Centre, Guildford, Fleetwood / Greater Surrey minus White Rock and Cloverdale = 2014 est. of ~350k, Vancouver 1951 was ~350k)
It can hard to know where to be patient and when to proceed, especially today when you can visit an area for the first time in ten years and it feels like a completely different place after a local development boom. We're all living in different cities mentally.
So here's some help stepping back and taking on a perspective different than the daily experience stored in your memory.
1935ish: Burrard Street bridge a few years after opening
• Video Link
1938: Lion's Gate Bridge construction
• Video Link
1950-1953: Granville Street bridge construction
• Video Link
1954: Drive across newly opened Granville Street bridge
• Video Link
1940: Broadway and Arbutus there had a dairy that delivered milk by horse drawn cart. Also by truck after wartime, but still a dairy. There used to be a huge golf course just south of there where Jimi Hendrix's dad worked.
• Video Link
There's also money and politics, which I realize is important, but I have no contribution to provide there.