Ugh. Who wants to climb a 20 foot steep hill just to get on a bikeway? I'll be down on the street, thank you.
Besides, anything either wide enough to be safe or with side fences and walls high enough to be safe would look horrendous.
Another thought-- when I imagine an elevated bikeway along, say, I-95, I can almost smell the exhaust...
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The bicycle, the bicycle surely, should always be the vehicle of novelists and poets. Christopher Morley