It looks like you are going over the top of existing turf. Could you scrape off the excess turf with a putty knife, so that the slabs of your road meet up better?
I have painted the edges of pieces like that black. Then it looks like tar in the joint.
You could also lightly tap the high spots down with a block of wood and a hammer until flush with the adjacent low section. It will crush the foam in the core.
I find that cheap paintable latex caulk is very easy to use for filling gaps in scenery.
Or just say it's a very bumpy road!
