Glastonbury Festival organiser Emily Eavis has apologised to festival-goers for traffic delays that have caused havoc on the opening day.
"Wet weather and ground conditions" are causing congestion in surrounding roads and stopping people travelling to the Somerset site.
An updated statement from the festival said they were "pleased to report that the picture is improving", but urged those who had yet to set off to "remain where you are for the time being, please".
"Because of the reduction in traffic approaching the area, queues are not currently getting worse and traffic is still making it on to the site," they advised in a Twitter message.
"If you have set off and have yet to reach the site, there is no need to rush. Take your time and stop off. The longer you leave it, the less you will have to queue."
Festival organisers issued a plea to people travelling by coach, asking them not to get off to try to access the site on foot.
"That is not a safe way to travel," they said.
Wiltshire Police confirmed that the A303 westbound, previously shut off after a four-car accident, was now reopened.
Avon and Somerset Police echoed the warning from festival organisers.
A force spokesman said: "We're strongly advising people to avoid travelling on main roads leading to the Glastonbury Festival site in Pilton due to heavy traffic."
AA president Edmund King also warned of "heavy and widespread congestion" on the roads around the festival site, including severe delays on the A361, A37 and A39.
Meteorologist Emma Sharples said the weather looked likely to clear up and predicted a dry night for festival-goers.
She told the Press Association that temperatures on Wednesday could reach highs of around 19C (66.2F) or 20C (68F).
The festival looks set to end on a high note with Sharples predicting a "fine, bright day with plenty of sunshine" and temperatures in the low-20s for Sunday.
Headliners this year include Adele, Muse and Coldplay, with reports also suggesting Will Smith and DJ Jazzy Jeff may be among the surprise guests.