Avenue-bridge not transitable

[Alvar 1]

I was trying to build an avenue bridge over the little piece of water here, however nothing is able to drive through the area marked with a red circle. What is the reason for this and would anyone have a good solution? The tram-avenue crossing just before the bridge isn't causing it, as I already tried without it. 

[A Nonny Moose 8,261]

To verify the pathing for that area you need to turn on the pathfinder's display.  This is done with the Extra Cheats DLL.  The command, after installing that mod is drawpaths.  It will show if there is a continuous route across that area.  The converse is hidepaths.

[dirtguru 24]

I was trying to build an avenue bridge over the little piece of water here, however nothing is able to drive through the area marked with a red circle. What is the reason for this and would anyone have a good solution? The tram-avenue crossing just before the bridge isn't causing it, as I already tried without it. 

As Nonny recommends, get the "extra cheats". In addition to the draw and hide paths function, extra cheats also includes a "terrain query" tool that for me is an absolute necessity.

Other networks react to slight changes in elevation differently, especially rail or in your case "tram". I have found that even a 1/10th of a meter elevation difference between a rail network and bridge approach will cause a pathing issue with either the bridge or rail when attempting to intersect that closely. May require bulldozing but I would try leveling a 5 tile square from the bridge approach, using the terrain query tool to assist in making each tile the same elevation as the bridge approach, then rebuild.

I have run into this same issue coming downhill to a bridge using RHW2. My region is full of hills and mountains intersected by creeks and rivers so I have had to adopt a set of specifications for roads and bridges. Among these is a spec requiring a minimum of two tiles of equal elevation from each end of a bridge approach for dragable roads. None required for puzzle piece bridges as I have not yet encountered any pathing issues when using them.