The charts above are from the office of the Texas Secretary of State. The top chart shows the in-person early voting for the first day in Texas for 2000, 2004, 2008, 2012, and 2016 in the largest 15 counties in the state. The second chart shows the percentage of registered voters that voted in-person on the first day in those years.
Note that both the number and percentage of in-person voters is up sharply. Part of this could be that people are getting used to early voting, but I think it also shows significant interest in this year's election -- and that many have made up their minds and are ready to get their voting done.
Early voting could drop off after this first day, but I don't think it will. I think we're looking at a larger than normal turnout this year -- both in early voting and on election day.
The charts below give the same information with mail-in ballots added.
