A new automatic field permeameter was designed to measure the in situ permeability of porous asphalt roads under field testing conditions. This apparatus works on the falling head principle; the falling head with time is measured through a porous road slab with a precise pressure transducer at the base of the apparatus at very close intervals of 100 (Hz) data points/s. With the falling head data, the specific discharge (υ) can be computed and plotted against the hydraulic gradient (ip1); the 3D flow permeability of the material is obtained at i p 1 >= 1 . The falling head data are shown to agree with constant head test data and are independent of the initial head at the start of the test. This demonstrates the repeatability of test measurements and the reliability of the test method, as it agrees very well with the wellaccepted constant head measurements. However, direct 1D permeability measurements of vertical and horizontal cored specimens from tested slabs showed the influence of permeability anisotropy on test results. Therefore, to make the 3D measurements useful for practical applications, a correction factor accounting for the effects of anisotropy, thickness, and size of slab on the test interpretation of the k value is obtained from a comprehensive finite element study, which allows for conversion from the 3D permeability to the effective 1D isotropic permeability. This is validated from the results of 14 porous test slabs (3 asphalt mixes and 1 portland cement concrete mix) where independent measurements of the 3D as well as the 1D horizontal and vertical permeabilities have been made, and excellent agreement is obtained when compared to finite element results.