The successful busways of the world fall into two categories. There are thick market busways, built in areas where there already is a thriving bus market, following the Magic Formula for Transit Ridership, and often earning a profit for the operators. Examples of this include Bogotá's Transmilenio, Guangzhou's Zhongshan Avenue line, and our own Lincoln Tunnel Exclusive Bus Lane and Bx12 Select Bus. Then there are former rail lines that are too narrow to fit more than a couple lanes of road, such as Pittsburgh's East and West Busways and Los Angeles's Orange Line.
Busways connecting sprawl with sprawl, without established bus markets and with heavy competition from government-subsidized roads, tend to have a hard time attracting passengers and fostering transit-oriented development. This 2002 post from Light Rail Progress cites Virginia's Shirley Busway and LA's Harbor Transitway as having lost riders, as well as former train lines like the Pittsburgh South Busway. At the very least they take a long time to build ridership, and are thus vulnerable to the Empty Lanes attack.
That makes it all the more mystifying why the Tri-State Transportation Campaign would push so hard for BRT across the Tappan Zee Bridge, and why they would happily throw the "prohibitively expensive" commuter rail option under the bus, so to speak. But it does explain why Tri-State pushed so hard for "full-corridor" BRT, from Suffern to Port Chester.
Work destinations of Rockland residents, according to the 2000 Census. From Tappan Zee Transit Mode Selection Report, Figure 1-4.
Nyack on the west shore of the Zee and Tarrytown on the east shore are both pretty small towns with high car ownership rates, and neither of them has enough residents or jobs to fill a busway between them when there are four parallel lanes of mixed traffic. Tarrytown has the Metro-North station where bus riders could transfer to Manhattan-bound trains, but even that isn't enough of an attractor. The buses could run in mixed traffic on both sides of the bridge, but that would limit their competitiveness with driving. Thus, transit advocates pushed for "full corridor BRT" to try to pick up as many Rockland residents and Westchester jobs as possible. And this, of course, added to the projected cost.
Given all this, the full corridor is pretty essential to the success of BRT. Without the full corridor you might as well not bother. In contrast, the highway widening plans put forth by the DOT were pretty clearly fantasy projects thrown in to make their bridge widening seem cheap by comparison. But because the bridge widening was only for the bridge, it also looked cheap by comparison to the BRT, and that's what allowed Commissioner MacDonald to reject BRT as not "financially feasible.”
Still, a billion dollars seems a lot for thirty miles of bus rapid transit, which we're constantly being told is "much cheaper than rail." So why aren't the proposals cheap?