The westbound side of the Caldera Highway (route 27) will be designated east bound for all the returning vacationers (from the Pacific coast towards San Jose) on Sunday. .
Please READ the info at the BOTTOM for specifics from the Tico Times.
While this will make your trip go faster, keep in mind if you need to take a ferry heading from the Nicoya Peninsula back towards San Jose, those ferrys are LIKELY to be experiencing super long lines heading back to San Jose. If not on a specific schedule, I'd wait until Monday to head back in that direction. (The ferrys will probably have long lines on Saturday too).
If you are on a public bus, I've always seen them make room for them. IF you are on private shuttles many (not all) companies drop you off at the ferry and you catch a different shuttle on the other side than this also probably won't be a problem for you (I've never seen them turn away walking on passengers), ask your shuttle company.
From Tico Times:
On Sunday, January 6, all traffic on Route 27 will go toward the capital to help those vacationing outside of San José make a quicker return from the Pacific region.
The change will force drivers traveling from San José to Caldera to use alternate routes.
The reversible lanes will be used on the stretch of the Caldera Highway (Route 27) connecting Pozón de Orotina (Alajuela) to Ciudad Colón (San Jose) from noon to 8:00 p.m.
Drivers heading to the Pacific during these hours should use Route 3 (Atenas-Aguacate-Orotina) or Route 239 (Puriscal-Turrubares-Orotina).