Solved a bug with the script: The order of the parameters was not correct.

launch/spawn_road.launch

@@ -25,7 +25,7 @@
         args="-param road_description -urdf -model $(arg road_name) -x $(arg x) -y $(arg y) -z 0 -Y $(arg yaw)">
   </node>
 
-  <node name="$(arg road_name)_set_road" pkg="road_description" type="set_road.sh" args="$(find road_description)/urdf/roads/road.png $(arg road_path)/$(arg road_name).png" />
+  <node name="$(arg road_name)_set_road" pkg="road_description" type="set_road.sh" output="screen" args="$(arg road_path)/$(arg road_name).png $(find road_description)/urdf/roads/road.png" />
         
   <node name="static_tf_$(arg road_name)_base_link_to_$(arg parent)" pkg="tf" type="static_transform_publisher"
       args="$(arg x) $(arg y) 0 $(arg yaw) 0 0 $(arg parent) $(arg road_name)_base_link 100">

scripts/set_road.sh

 #!/bin/bash
 
-if [ -f "$1" ]; then
-  rm "$1"
+if [ -L "$2" ]; then
+  unlink "$2"
 fi
 
-ln -s $2 $1
+if [ -f "$1" ]; then
+  ln -s $1 $2
+else
+  echo -e "!!!!!!!!!!!!!!!!!!!!!!! The desired road image file does not exist" 
+fi
 
 while true; do sleep 2; done