And it's probably one in which a lot of transit wasn't built.
Arguably, not building the southwest expressway kept the El around longer- if it had been built, I think what the plan was to have the Orange Line routed down alongside the new highway and going off somewhere on the Needham line. (Not sure what the plan was for other commuter rail/Amtrak- I want to say Fairmount though)is it possile we'd still have the washington el?
I don't see how that would be the case since the vast majority of Boston's transit system as we know it today was already in place before the Southwest Expressway was killed... notable exceptions being the Red Line extended from Harvard to Alewife, Red Line from Quincy Center to Braintree, Orange Line from Sullivan to Oak Grove - though none of these would have been as a result of the SW Expressway's cancellation.
I think ant is on the right path here. You can't look at this question in a vacuum. A counterfactual in which the Southwest Expressway was built is one in which the Inner Belt and lots of other planned highways would have been built. And it's probably one in which a lot of transit wasn't built. The scenario would make Boston an entirely different place from what it is today. Whether in a good way or a bad way, who knows. But the effects on traffic would be ancillary to the overall effect on the city.
The SW expressway plans called for a transit line in the same right of way.
If the SW expressway was built and the parking spaces were capped, as was done in the 1970s, the impact on traffic would have been to reduce congestion because there simply wouldn't have been enough parking spaces in the city to hold any additional cars driving into the city.
And if the inner belt had been built, it's likely that the big dig would have been simply replacing the central artery with an at-grade boulevard.
Such an outcome would have turned Boston in to Louisville.
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The red represents surface lots. A massive expansion of the capacity to bring cars in to Boston would have required a massive expansion of parking. No thanks, we already have too much parking.
And Louisville looks the way it does because their version of the Southwest Expressway wasn't cancelled.
If you live in the suburbs only drive in to the city to work and play you may not realize there are people who actually live there who don't want highways running through their neighborhood, no matter how convenient it may be for people just passing through.
I concur with Henry. The cap is a political limitation. It can disappear easily, given enough political pressure.