This is a misunderstanding of the issue. We aren’t talking about changing the road width here at all, just the lane number and direction. Most of these old, downtown streets were always narrow two-way roads. The difference is that oncoming traffic in the road and having only one lane in each direction dramatically lowers vehicle speeds, meaning these one ways are hugely more dangerous than the two way streets they replaced. In my city (which actually invented the downtown one-way!), 100% of pedestrians killed in the downtown area in recent years were killed on these one way streets. Yes, every single one.

To explain the history in more detail, when the field of traffic engineering emerged and it was decided that rate of personal vehicle flow was to be the sole design criteria for all roads in the US, narrow two way streets presented a problem. They couldn’t be widened due to adjacent development, and they were frequently congested and did not allow high speeds because of the close proximity of oncoming traffic. It was theorized that by converting them to multi-lane one ways, higher traffic flow would be accomplished.

As with other schemes to pack more vehicle throughput into urban areas, this seemed to work for a short period. It was deemed highly beneficial to the emerging political elites who had largely moved to the suburbs but worked in downtowns. Poor, minority, and other downtown residents who could no longer use or even safely cross the street on foot were deemed unimportant. Or more cynically, this danger and discomfort to the undesirables was another added benefit that might encourage them to leave the highly profitable business districts being built in their neighborhoods.

Before long, the new infrastructure for commuters enticed more and more suburban development and virtually everyone who could afford it moved out of downtown, and with this, traffic congestion increased dramatically. This is where the benefits of increased traffic flow disappear—adding more lanes in tight city streets where there are many intersections doesn’t really speed up traffic much at all when you reach even moderate traffic density due to all of the turning and lane-crossing required to navigate. Actually, this slows traffic and makes downtown streets safer during rush hour, but unfortunately they’re still extremely deadly at off hours, especially at night when fewer cars are on the road and visibility is poor.

In short, one way roads are dangerous and don’t really benefit anyone. In my city even local businesses have been agitating to get rid of them because they make it very difficult to pull over or cross the street to shop or eat. Eliminating them is a great and easy way to make city streets much safer and more pleasant without spending a lot of money or inconveniencing anyone.

The reason in theory is to allow multilane roads and higher traffic speeds. In practice this rarely improves things due to the high density of intersections and congestion in city centers. The real effect is just worse pedestrian and cyclist safety, in addition to making navigation much more difficult even for car drivers.