You might consider cross-posting your question on the CTPP listserv. More info here:
www.trbcensus.com/maillist.html This list is read by many transportation planners and modelers who are likely to have worked closely with mode split data.
Incidentally, another source for mode split data is the 2012-16 Central Transportation Planning Products (CTPP) which is derived from the ACS. You can learn more and access that data set at:
ctpp.transportation.org/ CTPP includes crosstabs and geography levels not available through the ACS as such.