I'm not sure why Broad Street to I-76 was built with 6 lanes, but 1990s traffic projections back in the early 1970s may have indicated that it would be adequate.
Traffic projections we now know, were frequently too low.
Exhibit A is a road I know very well, the Virginia portion of the Capital Beltway (I-495) between I-95 and the American Legion Bridge.
The Virginia Department of Highways (VDH), predecessor to the Virginia Department of Transportation (VDOT), was told by engineers and planners with the federal Bureau of Public Roads in the late 1950's or early 1960's that this part of I-495 would not be approved for more than two lanes each way (total of 4 lanes), and that's the way it was when it was completed in 1964. As I understand it, the feds were confident that there would be little or no growth in Fairfax County, Virginia, including a dusty crossroads intersection known as Tysons Corner at the junction of VA-7 and VA-123 and that four lanes would be enough for many years to come.
Those assertions would prove to be incorrect, and there were soon backups on the Outer Loop crossing the American Legion Bridge from Maryland to Virginia (the left lane dropped near the Virginia shoreline).
That went on until the mid-1970's, when VDH was able to secure approval to widen all of it from 4 lanes to 8 lanes in one massive project that was completed abut 1976 or 1977. Then the backups flipped, since the Beltway at the American Legion Bridge was still 6 lanes, so drivers headed from Virginia to Maryland had to endure years of misery (the Maryland part was only widened to 8 lanes (along with the bridge itself) around 1990.