When choosing to meet athletes on an endurance trail, the first thing to do is get as detailed of a paper map as you possibly can to chart the course out on a physical piece of paper. A GPS is not going to work in all areas. Cell phones fail and so do GPS systems on cars.
When plotting out the spaces, the first 10 miles of the trail do not require as
many trail stops as the distances will over 20 miles. From 20 miles forward, the SAG (Support and Gear wagon) should plan on an appearance every 3 to 4 miles.
That distance is going to depend on the safety of the athletes and where they can physically pull off away from traffic, or where a car can meet them.
Great places to stop are also places that have facilities like gas stations or parks with portable bathrooms.
If you are doing a road race that is going to put you against the freeway, then you are always looking to pull those athletes off the freeway into safety zones. Do not just stop on the side of the road or in a ditch with your emergency blinkers on. Look for places that you can turn the car off of the main road that is very light with travel – a dirt road is perfect, and will give that runner enough space where they can see oncoming traffic and spot the vehicle.
One of the things that I will frequently do is post a paper version of the road map on top of one of the coolers so that the athletes can track exactly where they are on the course, and they can have a mental image of when they will see the SAG again next.