After plenty of time here I have finally experienced enough of the police (not in a bad way) to properly inform you all of exactly how it works at this time.
Police will not randomly ask you for your passport. I have seen this on so many web pages, where they mention police or military will ask you for your passport if your a foreigner. It's simply not true. The locals say it was never that way, I don't know how it was prior to two years ago but it is simply not true anymore if it ever was.
I have walked past hundreds if not thousands of police and military officials, I've entered the airport plenty of times (Not departure section, but to let people off inside of the airport and the food court), I've gone into immigration offices and I have even been in a police station. I have never been asked for my passport or proof of residency here. Not a single time. I have also never been stopped by the police when driving.
The police do not typically pull cars over when they are in their vehicles. I have only seen one car stopped by a moving police car and that was after he hit another parked car, seriously. You have to do something insane to get a police car to pull you over.
The way that police typically stop you is with checkpoints. They set up a series of cones in the highway, road, wherever. You have to come to an almost complete stop to pass through them. If they see you were speeding or doing anything questionable, you will be flagged to the side.
They also randomly stop cars. They do not need probable cause. This is not Europe or the USA. They randomly select one car out of every so many to audit. I was in a car with a friend three days ago when this happened. The cop will simply flag you to the side, it does not mean you have done anything wrong. You will be asked for your license. In the case of a foreigner you will present passport and license. They simply take a look at it, walk to the back of the car and act like they are inspecting the license. Afterwards they return it to you and let you move on.
If the police find anything wrong, you simply pay a fine. They accept payment for the ticket usually (or to not write a ticket), either way... pay on the spot. You do not have the right to a fair trial. You are not going to see a judge and get out of the ticket. Foreigners and nationals alike: you are charged with speeding violations if the police say you are. I highly recommend not challenging it.
The police do not use radar guns. I have only seen two radar guns in years of being in El Salvador. I have seen thousands of checkpoints. I see road checkpoints almost everyday when I am driving. Some days there are more than others. It's not big deal as long as your not doing anything wrong. Police will "visually" clock your speed. Yes, it is legal. If they think you are going to fast than you are.
Speed is measured in Kilometers per hour here, not miles per hour. Many areas have signs which say "Max Kilometer per hour" much like a speed limit sign. Read into these very casually. As a general rule of thumb the police will not stop you unless your exceeding 100 km per hour. That is close to the equivalent of 60 mph. People fly past that but you can always stick around 100 km per hour to be safe. Drive to your right lane on the highways if you are not comfortable speeding up past that. Should you choose to drive slower in the heavy transit lane you WILL be run off the road. People are very aggressive drivers and they want to get to their destination.
Police will not write you a ticket for no lights, missing turn signals, missing hoods, missing windows, etc. You may get asked about it but you will not get a ticket. Likely you wouldn't even be flagged at a checkpoint for it and you would never be pulled over by a police vehicle for it.
Driving with alcohol is different here as well. Passengers in the car can drink all they want. They can be wasted. Front and back. The only person who can not be drinking is the driver. You can drink before you drive but there is a LIMIT. The limit here, well who knows. Basically it is if the cop feels your over it. I want to make it very clear that DUI is not relaxed here. Cops will almost always think your over the limit. If you have had more than two beers, do not drive here. It doesn't matter who you are, if the cops catch you drinking and driving you will be taken to jail. Minimum is fifteen days at this time, you go straight to jail, do not pass go and do not collect $200.
Jails in El Salvador are not fun. They do not care about your personal rights. You will be treated like a criminal and it will not be fun. YOUR EMBASSY will not help you. The American Embassy does not assist if you are in criminal situations. Already have checked into that for a friend, they will not get involved.
Police generally will not ask for a car registration when they stop you. Registration comes in the form of a tiny card that looks like a license. It is purchased once a year for $28.00. Value can be a little higher if you have a truck or other type of vehicle. Insurance is not required and very few have it therefore you will not be asked for that either. Usually just a license.
Police travel in four forms. First is in modified pickup trucks. The second form is in small police cars. The third form is on mopeds and motorcycles. Finally police travel on foot. Most commonly they are in police pickups. Every now and then you will see a police car. Tons of police are on foot and there are plenty of police motorcycles. Police motorcycles are much more likely to stop a moving vehicle (although still rare).
In some zones you may find "Tourist Police", they will be on bicycle sometimes. They are just there to make sure that tourist zones are protected. The national police are the "PNC". Do not confused security guards with police. They are uniformed, often have fun belts and carry weapons. They are privately hired by businesses to protect the business. The police are in darker uniforms with caps. Police here are generally friendly unless you give them a reason not to be. They are especially friendly to tourists however that doesn't mean the law won't apply to you.
For emergencies in El Salvador, dial 911. Make sure you identify a street name or a landmark since they won't automatically know your location here.
Visit the web page of the PNC (Policía Nacional Civil) here.
(Police Pickup - Most common)
(Police Car - Less common)
(Police Checkpoint - Newer Pickup (rare))