How do roads generate traffic? Simply put, building a new road or expanding an existing road can make cars on a specific route simpler, faster and more time-saving. For this reason, it attracts people to use cars instead of other inconvenient means of transportation, such as walking, cycling or public transportation. It also attracts people who used to choose other inconvenient routes, and also attracts people who don't travel during peak hours.
Downs-Thomson Paradox-Wikipedia, together with better roads and fast travel, invites people to stay away from work and school shopping (otherwise, invites people to stay away from home to work, study and shop), so a brand-new trip did not exist before or was greatly shortened. In some cases, it may even be different from forcing you to use a car before an unavailable alternative means of transportation is available, even if you are happy to travel: for example, if you want to go to the other side of i- 10 Avenue, you will drive to the next interchange, which is better than trying to walk through it. This is a car imprisonment, a problem for the rich, not a traffic imprisonment, but a problem for the poor.
This feedback effect between cost and demand is very common in economics: when the price of a service or commodity falls, more people will want to buy or use it. If access time is included in the cost, reducing access time will increase the number of people using the service.
No one will be surprised that a convenient restaurant offering delicious food is waiting in line outside. No one will be surprised if the restaurant owner expands the venue, adds more tables and more people go to the restaurant. No one will be surprised when the expanded team is formed again.
To many people's surprise, the same thing happens on roads: better roads make driving more convenient and attract people to drive. People will drive more, take less public transport and walk less. One side effect is that the service provided by public transport will deteriorate, because it has lost too many customers to maintain, and the municipal government's investment in sidewalks and bicycle lanes will be reduced, because people no longer walk or ride bicycles. This makes the replacement of cars more inconvenient and attracts more "customers".
This is the end of the "hydraulic analogy" mentioned by Paul Barwick in his answer: if your sewer is blocked, you can install larger pipes to avoid blocking. But this will not lead to more sewage in your home. Economists will say that in this case, "demand is rigid" and will not respond to changes in supply. As far as roads are concerned, demand is flexible, just like restaurants, responding to changes by increasing or decreasing supply.
So what happens if you reduce the size of the road? Soon, you will see more congestion. In the long run, if the reduction of roads is accompanied by measures aimed at making the selected trips more convenient (bus lanes, bicycle lanes, better sidewalks ...) and urban planning measures aimed at making long-distance trips shorter (mixed use areas ...), then some people may decide that it is inconvenient to use cars and choose different modes of travel.
Because the space occupied by a person in a car is at least 15 times that of a person who can get on public transport (permanently, because the car needs to stop at the origin and drive at the destination), persuading people to use public transport (bicycle, walking) can save a lot of space. This in turn reduces the necessity of driving, makes other modes of travel more convenient, and attracts more people to stay away from cars.
Therefore, this is the way to reduce traffic congestion by narrow roads. This policy is called "Road Diet", named after the famous words of lewis mumford, a city planner and sociologist. He once said: "widening roads to deal with congestion is like relaxing belts to deal with obesity."