(See image below - and note, you can click the image to get a bigger version.)
Polk is also one of the few streets that provide full cross-town connections across Downtown to connect with neighborhoods beyond (shown as blue lines). Polk connects the East End all the way to Montrose. Navigation and Harrisburg are the only other streets that provide good connections, but between entirely different neighborhoods in downtown and on either side.
Between Leeland and Texas, a distance of 5/8-mile, Polk is the only street in addition to Leeland and Texas that provides a significant connection. In most part of town, major roadways are space a minimum of 1/2-mile; in most parts of town, they are closer to 1/4-mile.
The remainder of streets have challenges:
Commerce. Dead-ends 1.2 miles to the east at Drennan Street and 0.6 miles to the west at Milam Street. As such, it does not provide a deep connection into the East End.
Leeland. Provides a connection deep into the southern parts of the East End; however, it dead-ends at Louisiana Street 0.8 miles to the west. It also must cross the busy West Belt freight rail line at-grade, and all traffic is frequently blocked by passing and stopped freight trains.
Pease. Mainly provides cross-town access for IH-45 traffic, directly serving ramps east of Emancipation Avenue and west of Brazos Street. East End drivers can access Pease via the Gulf Freeway frontage road, but that involves high speeds and a confusing intersection at Emancipation.
Pierce. One-way eastbound only; provides access to far south side of Downtown and the East End. There is not perfect westbound pair.
Pierce Elevated (shown in dashed lined). This is an existing connection between the East End, Downtown, Montrose, and other neighborhoods west of Downtown; however, TxDOT is proposed to remove the Pierce Elevated.
The short answer is: we don't know. There have been no publicly-available traffic analyses released that show the impacts of abandoning Polk. However, there is a good chance that it will negatively impact many people, with an exceptional impact on some.
Permanently closing a street will always redistribute traffic onto other streets, so we can generally expect traffic to increase on the few other remaining streets into and out of Downtown. This will likely be especially significant during the arrivals and departures of large crowds to basketball games at Toyota Center, baseball games at Daikin Park, soccer games at Shell Energy Stadium, and conventions at the GRB.
More specifically, this will certainly impact people coming to those destinations from the southwest along I-69 and south along 288. (See maps below - and click to be taken to the Google Map!)