You can find information about it here under the heading "Sommargata på Vätterstranden i centrala Jönköping".

https://www.jonkoping.se/trafik--stadsplanering/planarbete-och-samhallsbyggnad/stadsmiljoprogram-och-arkitektur/tillfalliga-omvandlingar-i-stadsmiljon

It is signposted with a “påbudsskylt" D4 and an unbroken line that forces you to turn to the alternate route along Östra Storgatan.

Try to avoid removing things that you don't know if it is correct or not in the future. 