There really is no way to avoid some drivers. While stopped, it doesn't matter how many flashing lights you use, people will still hit you. There can be 2 police cars, 3 fire trucks and an ambulance at a car accident and people still end up hitting one of the fire trucks blocking traffic. How does someone miss a 30'+ long truck with tons of flashing LED's and an entire back end covered with reflective chevrons?
Cones can work okay but there is a certain method to cones. If you check out one of the DOT sites you will find a method to setting out cones. For example if you are parked on the side of a road where the speed limit is 35mph, you need to start setting cones out say 200' behind the rear of your vehicle and have them spaced 10' apart (those are not the actual numbers used, but you get the idea). Placing 3 cones 10-20' behind your trailer really won't do much, if a driver hits the first cone, he really won't have much time to react (even if he realizes he is about to hit your trailer) since you only gave him 20' behind your trailer. Your only hope is that the driver sees these bright orange cones from down the street and takes notice of them before its too late.
My opinion is that if you are working on a busy road or faster road, park in the customer's driveway so you are out of traffic. Or if there is a large enough shoulder, make sure to pull off the road entirely.
'07 Chevy 2500HD
2010 John Deere Z925A 54" MOD
2011 John Deere Quik Trak 647A
John Deere walkbehind