  We use the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) to
reduce the uncertainty in the local value of the Hubble constant (H_0) from
3.3% to 2.4%. Improvements come from new, near-infrared observations of Cepheid
variables in 11 new hosts of recent SNe~Ia, more than doubling the sample of
SNe~Ia having a Cepheid-calibrated distance for a total of 19; these leverage
the magnitude-z relation based on 300 SNe~Ia at z<0.15. All 19 hosts and the
megamaser system NGC4258 were observed with WFC3, thus nullifying
cross-instrument zeropoint errors. Other improvements include a 33% reduction
in the systematic uncertainty in the maser distance to NGC4258, more Cepheids
and a more robust distance to the LMC from late-type DEBs, HST observations of
Cepheids in M31, and new HST-based trigonometric parallaxes for Milky Way (MW)
Cepheids. We consider four geometric distance calibrations of Cepheids: (i)
megamasers in NGC4258, (ii) 8 DEBs in the LMC, (iii) 15 MW Cepheids with
parallaxes, and (iv) 2 DEBs in M31. H_0 from each is 72.25+/-2.51,
72.04+/-2.67, 76.18+/-2.37, and 74.50+/-3.27 km/sec/Mpc, respectively. Our best
estimate of 73.24+/-1.74 km/sec/Mpc combines the anchors NGC4258, MW, and LMC,
and includes systematic errors for a final uncertainty of 2.4%. This value is
3.4 sigma higher than 66.93+/-0.62 km/sec/Mpc predicted by LambdaCDM with 3
neutrinos with mass 0.06 eV and the Planck data, but reduces to 2.1 sigma
relative to the prediction of 69.3+/-0.7 km/sec/Mpc with the combination of
WMAP+ACT+SPT+BAO, suggesting systematic uncertainties in CMB measurements may
play a role in the tension. If we take the conflict between Planck and H_0 at
face value, one plausible explanation could involve an additional source of
dark radiation in the early Universe in the range of Delta N_eff=0.4-1. We
anticipate significant improvements in H_0 from upcoming parallax measurements.
