They didn't have jet fans back then, not sure when the first was installed, but part of what made them feasible was vast reductions in motor vehicle pollutants enabled by advancing technology.
I think the first "jet" fan I saw in the U.S. was on the PTC's "new" Lehigh Tunnel (the tube that normally carries southbound traffic (though I think PTC can run two-way traffic in most or all of its tunnels if needed, as the crossover ramps are there).
Getting back to the BHT, I do not think MDTA is going away from its "traditional" approach to tunnel ventilation using plenums and fan buildings.
Anyone looked at the plans for the CBBTD's new Thimble Shoal Tunnel?
Hampton Roads area tunnels all followed the "traditional" approach to tunnel ventilation, until the ERT project where all 4 tubes have jet fans. I'm not sure whether the various costs and benefits, on the balance, are substantially different or whether they are close.
I don't see detailed CBBT plans on their website, but they do have a cross-section here --
http://www.cbbt.com/project-description/The finished product will look more modern compared to its ’60s-era companions. It will be similar to the new Midtown Tunnel, completed last year, with bright LED lights, hanging electronic signs, jet fans and a separate fire exit corridor.