The red shows where the object blocks routing- it will keep Sims from walking there. The road lets Sims get out of their cars and 'materialize' on the sidewalk. That can place them on the sidewalk where the object blocks passage. It traps them. It might never happen- because they'd stop the car at a lot but sometimes thing work to mess things up if there's a chance. :)
Give horses 2 squares of passages around objects and lots to pass (in those open areas). Sometimes they seem to spawn in the strangest places. :)