I just realized I was wrong about something on Oak that's a key detail and it explains a lot.
US 71 to downtown didn't exist in 1997, it wasn't finished as a road until 2001, so it wasn't that long ago that Oak would have been used for inbound traffic from the south. Get off 71 at US 56, turn north onto Troost, to 18th to Oak.
This traffic count data gives us numbers each direction on Oak (go to the page with line 470) that's better than my anecdotes.https://data.kcmo.org/Traffic/Traffic-C ... /s7kw-x59n
At peak morning hours you can see there's now more cars heading SB until you hit 19th. You can see a lot more EB traffic on 19th than 18th and more WB traffic at 18th. So it appears most cars in this area take Oak SB and enter the crossroads at 18th. This makes sense, 18th is a 35mph road until just west of Main so it would get more use. I would guess 19th EB is traffic is drivers taking Oak from 22nd and turning to double back south to the Children's Mercy lots because it would take longer to get off at Truman.
Gillham at 27th has more SB traffic in these counts, showing this trend continues south through the end of downtown. Holmes/Truman has less cars WB than Oak and Truman, showing more cars take Truman after the one-way split than before it, and Oak is the major SB street in this area.
To be fair the numbers don't differ by much, it's not a 3:1 ratio, but they consistently show more SB traffic.
I checked google street view and the 1800 block didn't have a 9-6 pointing to a 9-4 sign. It was a 7-4 sign pointing to a 7-6 sign. So that one block allowed parking during peak hours already. The 1900 block didn't have any hours control, it was a 24-hour parking zone that ended where 20th would be at. Both blocks were already signed for two lanes SB
If we look at Truman to 18th SB the signs disallowed parking 4-6. So it's signed for outbound traffic in the afternoon coming from the CBD expecting people to turn at 18th
NB north of 19th was no parking 7-9, one hour 9-6. South of 19th was no parking. So it's signed for inbound traffic in the morning going to the CBD
It appears the city never flipped the no parking hours to adjust for changing traffic patterns after US 71 opened. It doesn't matter what parking enforcement there was or wasn't because for more drivers parking was always allowed in the outbound lanes at peak hours.
We're finally seeing the traffic control changed for a post-US 71 environment. So when this change is underwhelming it's because it's 15 years behind.