From the ERC website forum (via Jonny May):
I've just received a message from Musselburgh Running Club warning of their race on Saturday 20th July. It might be a good idea to avoid the bridge until noon that day:
"Good morning. I'm from the Musselburgh Running Club. It's our annual 10k on Saturday 20th July and we have a record entry of 400 runners who'll be coming from near and far. As you doubtless know there are significant issues with the use of the Electric Bridge at the end of New Street. It's looking likely we will have to use the footbridge both ways for our race. We would ask you to let you members know about the inevitable congestion this may cause and indeed it may be advisable to avoid that area until noon on the 20th. Thank you for your understanding."
Note: It's also the Open at Muirfield that weekend and it may also be an idea to avoid Gullane and surrounds - http://www.eastlothian.gov.uk/info/760/events_and_festivals/1521/the_open-travel_information/6