|Probably not what Jacobs had in mind (source)|
Jacobs was far ahead of her time in writing that widening and building new roads instigates additional automobile traffic. This is an observation that had only begun to be discussed in the 60s, and people still have difficulty with it today. In 1962, Anthony Downs proposed "the fundamental law of highway congestion" which states that highway travel increases closely in proportion to highway construction. There is also a similar proposition called the Lewis-Mogridge Position. Recent work also explores this relationship in more depth.
Nowadays, responsible engineers and planners (ones not trying to profit from excessive concrete) understand this dilemma and in some places, have actually tried some different ways of fighting traffic that Jacobs would appreciate. But many people get confused when you discuss this issue with them for the first time. It is counterintuitive. Most people have experience driving on a highway that has one lane shutdown for some reason. They see the bottleneck created and exclaim that narrowing the highway permanently could only make things worse!
The confusion lies in the various different perspectives one can take on this problem. Most people are thinking in the immediate situation: the cars on the road are stuck because the highway is suddenly smaller than it was before. But that's not the perspective that road designers and city planners are supposed to take. They are supposed to think about how each road plays into the larger network of transportation, and the even larger world of development and land use, over the course of years. The construction (or removal) of highways and roads causes many people to re-evaluate their patterns of travel, their choices of residence, and how they go about getting around. It is this effect which causes observations like "fundamental law of highway congestion" to come true.
Let's look at an example: Boston's inner belt highway famously has no congestion. That's because Boston's inner belt highway doesn't exist. True, there are roads where the inner belt highway was planned, and they are often congested. But if the highway had been constructed, it too would have become congested in time, much like Rt 128 is today. And its construction would have ravaged a great swathe of land through Roxbury, Fenway/Kenmore, Brookline and Cambridge. In fact, the spot I am typing this from would have been badly affected, possibly destroyed. Thankfully, the residents of the time successfully opposed the highway. The ones that did get built have done a lot of damage, however. After all, we just spent $22 billion to bury one of the most infamous highway projects of all time. And one of the major unintended effects was to create even more congestion at the choke-points entering the city.
Fundamentally, congestion is going to happen around any attractive location, and there's almost nothing you can do about it. Automobile congestion springs up easily because cars are a particularly space-inefficient form of travel. If one person decides not to drive downtown, it is likely that somebody else is waiting to take up that space. And if more space is made available, then more drivers will take advantage of it. That's why public transportation doesn't cure congestion, and it is dishonest to claim it will. What public transportation can do is accommodate greater numbers of people than the roadways will permit. Although the congestion doesn't go away, the absolute number of people able to travel can go up, given appropriate transit options.
Only road pricing seems to have any effect on congestion. This shouldn't be too surprising to anyone who supports free markets, but many people go slightly insane when confronted with their addiction to Road Socialism. If a product is free or underpriced, then the supply becomes exhausted and queues form to obtain it. Most roads are free, or underpriced, so it is only natural that people form queues of traffic trying to use them. When road pricing is done right, it doesn't discourage travelers, it just sets a market incentive to use other modes of transportation when the roads become too busy. Sadly, it seems like it will be quite a long time before this reform is implemented around here, as it requires people to give up what many feel is an entitlement. If people could come to understand that "time is money" and either way, they're going to pay, then perhaps those stubborn impulses can be checked.