All went as planned as the massive Commonwealth Avenue bridge project got underway in Boston Friday morning.

But as state highway administrator Jonathan Gulliver warned, "It's early."

The replacement of the western side of the bridge will take two weeks. During this time, the section of Commonwealth Avenue near Boston University is closed to all through traffic — only MBTA busses, shuttle busses replacing Green Line trolleys, pedestrians and cyclists are allowed.

The BU Bridge between Boston and Cambridge is also closed to traffic.

Most seemed to take the changes in stride, though some motorists appeared perplexed by thelabyrinth of detours in place. But there are many police officers and transit personnel on hand to provide advice and guidance. Transportation officials say anyone planning to drive in the area over the next two weeks should use whatever GPS devices they have and consult the DOT web site to see what detours they may face.

Traffic was unimpeded along the Mass Pike under the bridge Friday morning. But that is expected to change dramatically starting at 9 pm, when the number of lanes will be reduced from 8 to 3. It will stay that way for the duration of the two-week project.

Officials say that's where we could see significant traffic disruption.

Some fear backups on the Pike could stretch east to Logan Airport and west to Wellesley, especially during Monday morning's commute, if people don't heed the warnings to stay away if at all possible.

Commuter rail service from the west will not be affected during the week — in fact, some extra service will be provided from Framingham. Buses will replace commuter rail trains on weekends for the project’s duration.