Welcome in the world of making roads =)
Aaah memories, this is one of the first challenge I met when I started working on trackmania. I remember it again. It was for the earliest version of the Rally DirtRoad (almost 10 years ago, oh my god!

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I didn't find the answer very fast to be honest.
Your corner must be a bended "finished straight", don't make UVs after bending but before.
There are different tricks, the first one is to mirror your UVs at the middle of your corner (you avoid seams with the following block), then deform it to have smaller UV inside the corner and bigger UVs outside. This may looks odd sometimes, but you will avoid texture stretching and UV seams.
Rectangular UVs on trapezoïd shapes is most of the time not very nice looking.
At the end, the UVs of your corner should look more like a triangle or a boat sail than a rectangle.
The most advanced need special materials such as the ones used on the GP roads (RoadRace and RoadRaceTrace), but you will often nned that mirror/triangular scale trick to avoid stretch & seams.
Also as suggested TMarc You can do a specific texture dedicated to your corner...However, this cost memory, is not easy to do and takes time; and what will you do when you'll have to do bigger curves, TShaped or Cross sections, or diagonals?
I think the dedicated road shapes textures have to be used wisely. (unless you have unlimited GPU memory
Hope this may help.
Fix