Most advice about breaking down on a highway is written by people who have never done it. It tells you to stay calm and call for help, which is true and useless. What follows is the order the decisions actually come in, and the reasoning behind each one, so that you are choosing rather than reacting.
Keep the car moving as long as it will move
A vehicle that is still rolling is worth more than a good parking spot. If the engine has cut but you still have momentum, that momentum is the only thing you control, and you spend it getting right and getting forward. An exit ramp, a gore area, a wide shoulder past a bridge, a gas station: any of those is worth several hundred feet of coasting.
Power steering and power brakes go heavy when the engine stops. The steering still turns and the brakes still stop, both take much more effort than you expect, and the surprise is what puts people into the next lane. Push harder, and do not shut the ignition fully off while you are steering, because on many cars that locks the column.
Signal before you move. Drivers behind you have no idea anything is wrong, and the difference between a car changing lanes and a car failing is invisible from back there until the closing speed makes it obvious.
Choose the spot in the second you have
The shoulder is not one thing. A straight, wide, level shoulder on a rural interstate is a place you can stand. A narrow shoulder inside a curve, on a bridge, in a tunnel, at the bottom of a hill or beside a concrete barrier is not, because the sight line that would let a driver see you is the thing the geometry has taken away.
If you have any choice at all, stop where a driver coming up behind you has a long look at your vehicle. Past a curve, not inside it. Beyond the crest, not on it. Off the bridge deck rather than on it, even if that means driving on a flat tire and destroying the wheel. A wheel is cheaper than every alternative.
Get the wheels off the paint if you can. Every foot between the driver's side of your car and the travel lane is a foot of margin for someone else's mistake. Turn the wheels away from the road, and set the parking brake on any grade.
Hazard lights, then the decision about staying inside
Hazards go on the moment you commit to stopping, and stay on. At night, leave the headlights on as well if the battery allows it; a lit car is read as a car much earlier than an unlit one.
Then decide, quickly, whether the car is a shelter or a target. Inside the vehicle with seatbelts on is the safer choice when the shoulder is wide, the sight lines are long and traffic is light. Outside and well away is the safer choice when the shoulder is narrow, the road is fast, visibility is poor, or you are anywhere a distracted driver would drift into you. Say it out loud to whoever is in the car with you, so nobody opens the wrong door.
Everyone leaves through the doors away from traffic
If you are getting out, nobody uses the driver's side on a right shoulder. That includes the driver, who climbs across. It includes the person who wants to grab something from the trunk, which is the single most exposed place to stand on the entire vehicle.
Children and pets come out on the same side and are held. A dog on a shoulder is a dog in the travel lane a few seconds later. Take phones, keys, wallets and water, and leave everything else.
Where to wait is not next to the car
Walk up the slope, over the guardrail if there is one, and back from the vehicle in the direction traffic is coming from. Behind a barrier is what you want. Up an embankment is the next thing. Standing in front of the car, leaning on it, or sitting on the trunk are all ways of being where the car will be pushed if it is struck.
Do not walk along the shoulder toward an exit unless the alternative is worse and the distance is genuinely short. Walking on a highway is more dangerous than waiting on one, and a person on foot at night is close to invisible to a driver at highway speed.
Reflective triangles or flares belong behind the vehicle, well back, on the shoulder, and only if you can place them without walking into a lane. Setting them out is not worth stepping into traffic for.
Calling, and what the person on the phone needs
Call the number on the highway signs if there is one, or 911 if anyone is hurt or the vehicle is in a lane. Then arrange the tow or the roadside call.
What dispatch needs is location, in this order: the route number and the direction of travel, the nearest mile marker or exit number, and then the city. "I-95 northbound, mile marker 118, past exit 116" is a dispatchable location. "On the highway near the mall" is not, and it is the reason a truck goes to the wrong side of a divided road and starts again. Mile markers are on small green posts on the shoulder; the number at the top of the exit sign is the other usable reference.
Then say what is wrong, and be honest about it, because the answer decides which truck comes. A car that will not start needs a different vehicle than one with a wheel off the pavement, and a car that does not roll, steer or brake needs a flatbed and possibly dollies. Say if it is all-wheel drive, lowered, in a parking structure, or up against a barrier. Say how many people are with the vehicle, because a two-seat truck cab decides whether a second vehicle is needed.
Nobody can promise you a time, and you should be suspicious of anyone who does
Traffic, weather, and where the nearest available truck happens to be all decide arrival. We connect you with a vetted, licensed local operator, and the operator confirms the price with you before anything is dispatched. What we will not do is give you a number of minutes from a web page, because nobody on a web page knows where the truck is.
If the police call the truck, you are not the customer
This is the part almost nobody explains. When a vehicle is blocking a lane, sits on a shoulder longer than the state allows, or is involved in a collision, the police can order the tow from a rotation list. At that point the operator is working for the agency, not for you, and the rate is whatever the local ordinance or the state schedule says it is.
Those two examples are what a regulated tow looks like: a published maximum, a mileage rule, and no negotiation at the roadside. There is no national schedule and no national average worth quoting — every state and many cities set their own, and the only number that governs your bill is the one in force where the car stopped. It is worth knowing which of the two situations you are in before you start arguing about price, and worth asking for the itemized invoice either way. How each line on that invoice is built, and which ones can be challenged, is covered in how to read a tow bill.
Before the truck arrives
Take photographs of the vehicle where it sits, from far enough back to show the lane and the shoulder, and close enough to show any existing damage. If the car is towed, those images are the record of its condition at the moment it was picked up.
Have the registration and your ID out. Decide where the car is going before the driver asks, because the destination changes the price and a decision made through a truck window is a decision made badly. If you do not have a repair shop, say so and ask what the storage arrangement is and what it costs per day.
Finally, take the plate and the mileage down, and keep whatever paperwork you are handed. A tow that goes smoothly needs none of it. The one that does not is decided entirely on documents.