Highway shoulders are dangerous. Drivers struck by passing vehicles while waiting for help is a documented annual fatality category in transportation statistics.
All US states have move-over laws requiring drivers to change lanes or slow down when passing emergency or service vehicles on the shoulder. Compliance is poor.
Behavior that increases risk: standing next to your vehicle on the traffic side, walking along the shoulder, working under the vehicle on the shoulder.
Behavior that decreases risk: staying inside the vehicle if shoulder is narrow, exiting on the non-traffic side, walking 20+ feet from the road, deploying triangles or flares behind the vehicle to warn approaching traffic.
If a service vehicle is on scene with you, position yourself on the opposite side of the service vehicle from traffic — let the service truck shield you.