A day after a freak accident backed up traffic for miles and forced motorists into lengthy detours, southbound Interstate 75 through South Sarasota County is expected to reopen for most of today, including the morning rush.  But repair work on the bride damaged by the dump truck Wednesday will close the heavily traveled southbound lanes between River Road and Sumter Boulevard again from 7 p.m. until early Friday. Similar nightly closings are planned through the weekend on the nine-mile stretch of southbound lanes.