We report dual-frequency VLA and multifrequency VLBI angular broadening measurements of a number of radio sources located along the Galactic plane in the longitude range l = 20-degrees-80-degrees and with Galactic latitude \b\ < 0.5-degrees. We use the lambda-2 dependence of scattered angular size to distinguish between intrinsic and scattered source structure. Our observations support a previous report that the structure of the extragalactic source 1849+005 is dominated by interstellar scattering. This source appears to be one of the most heavily scattered sources known. We have also observed angular broadening for the sources 1855+031 and 2008+33D. Five additional sources, 1905+079, 1922+155, 1932+204, 1954+282, and 2001+304 show indications of being affected by interstellar scattering. For the remaining sources we obtain upper limits to scattering with the value of the upper limit depending on the observed source structure. We find that the observed scattering in the Galactic plane decreases with increasing Galactic longitude, reaching a minimum at l almost-equal-to 60-degrees and then becoming prominent again in the Cygnus region. The magnitude of scattering in the interarm region between l = 50-degrees-70-degrees is comparable to that predicted by a model calculation of a "smooth" component of interstellar turbulence. The more intense "clumps" of interstellar turbulence may therefore be mainly confined to the spiral arms of the Galaxy. A computer simulation was undertaken to find clump properties consistent with our observations. This simulation indicates that a complex distribution of clumps is necessary, with a range of scattering strengths, and that the form of the clump distribution as well as the density of clumps must change with increasing galactocentric distance. The simulation constrains the z scale height in the Cygnus region to be approximately 40-70 pc. Although there is a weak correlation between scattering and H166-alpha-emission measure, interpretation of this result is unclear and leaves open the identification of the regions of enhanced turbulence.
