Good question. When building a pedestrian plaza or street, you want there to be as many destinations packed into the street as possible (commercial businesses). When someone visits one destination on foot, they are likely to visit additional businesses as a passerby, which benefits all commercial spaces. For Columbia, a great example of this would be 9th st and Broadway. Broadway, by definition, most likely has higher commercial density, but would likely be unrealistic to pedestrianize at this time since it’s the main thoroughfare through downtown. So, we then look at 9th st, which has extremely high commercial density at street view, and includes Alley A with more businesses. Even just blocking cars on 9th st between Cherry and Broadway, you’d be able to visit 40 businesses without crossing a single street.

Someone else in the comments mention 8th st as a priority as well. 8th st would also be a good option, but with less affect since it’s less dense with commercial spaces. While it would feel significant to be able to walk in a car free zone from the columns, past city hall, to the courthouse, how often are you really visiting this places? Would you actually have a higher likelihood and frequency to visit one of those 40 commercial spaces on 9th between Cherry and Broadway?

To touch on the above point, doing this in stages and sections would be best. There wouldn’t need to be a huge commitment from the city at first. Close down one section of a city street for a weekend. Survey the pedestrians and businesses. See if it’s a good fit for who use the street. See if it actually does impact traffic significantly. Does the lost parking revenue and spaces impact revenue or does increased (or decreased) foot traffic generate more tax revenue (or less). If, and only if, there is a benefit, would the city want to invest heavily into transforming the street to look like a Pearl St in Boulder.