Headlights
Short single flash of high beam — a signal to attract attention. If they flash from behind while driving on the highway — it's a request to yield (for this, it's also appropriate to use the left turn signal — more on that below).
If you don't plan to yield — lightly press the brake pedal so the brake lights come on for a moment. If they flash from behind but you can't change lanes right now (while ready to let them pass) — turn on the right turn signal, which will mean: "I'll yield as soon as possible."

If they flash from the side — they're asking you to let them merge into dense traffic, coming from a secondary road or changing lanes.
One short flash (sometimes two) is also interpreted as: "go ahead, I'm letting you pass," and when driving towards each other, one flash can serve as an expression of gratitude.

In the dark, one short flash of high beam may mean: "turn off your high beams — they're blinding me." In that case, you should switch to low beam or turn off fog lights. If you're driving with low beam on — respond with a short flash of high beam, meaning: "I have low beam on, not high."
Two short flashes of high beam — danger ahead (sometimes they mistakenly flash only once).
Danger can vary (in post-Soviet countries, this signal is most often used to warn about traffic police posts). In any case, it's worth slowing down and being more attentive.

It's customary to thank for warning about danger — with a raised open palm or a nod of the head. Note: not with a return flash, but with a gesture. This tradition also comes from Soviet times: if you were warned about a police ambush, a return flash could attract the attention of inspectors who would stop you for "improper use of light signals."
One long high beam signal — a demand to pay attention. If they flash from behind on the highway — they persistently ask to yield.
A long high beam signal also means "I can't (or don't want to) let you pass" when passing. Sometimes it's accompanied by a sound signal.
Hazard lights and turn signals
A single (better — double) flash of hazard lights — a sign of gratitude or apology — depending on circumstances.

Hazard lights are also turned on during forced lane changes (this is both a request to let pass and an apology for inconvenience).
Turn signals — a multifunctional tool.
If the car ahead turns on a turn signal but doesn't change lanes — it's a signal that there's an obstacle on the road on the side where the turn signal is on.
Accordingly, a left turn signal on the car ahead means: "overtaking is prohibited, there's an obstacle ahead" or "I can't yield."

A right turn signal — "overtake, the way is clear" (but make sure the driver isn't just about to turn).

Most often, such signals are used by drivers of large vehicles (trucks, vans), whose size significantly limits the view of those behind.
Oncoming cars, warning of danger, may supplement double high beam flash with a turn signal indicating which side to expect danger from.
A driver moving in the left lane, turning on the left turn signal, asks the driver of the car ahead to let them pass.

Sound signals
Recall: according to clause 9.5 of the traffic rules, a sound signal in the city is allowed only to prevent an emergency. That is, expressing emotions with the horn is not allowed.
Long honk/honks — persistent attention to prevent an accident; a sign of extreme displeasure.
One short honk — attracting attention, as well as a sign of gratitude or slight displeasure; two and three short honks — evidence of growing displeasure respectively.
Gestures
In dense traffic, wanting to change lanes or turn, a turn signal is sometimes supplemented with an outstretched arm with an open palm.

As a sign of gratitude for warning about danger from an oncoming driver — nod head or raise hand with open palm.
A neighboring driver points to the roadside — you have a malfunction, need to stop; slaps palm in the air — trunk is open; hand describes a circle and points down — tire is flat; describes a circle, forming a fig with the hand — a stone is stuck in the wheel (relevant for trucks).
Gestures replacing turn signals (used when they are faulty):
- turning left, as well as U-turn, lane change left, starting from the right edge of the roadway — signal is given with left arm extended to the side, or right arm extended to the side and bent at the elbow at a right angle;
- turning right, as well as U-turn, lane change right, starting from the left edge of the roadway — signal is given with right arm extended to the side, or left arm extended to the side and bent at the elbow at a right angle.
If the brake system is faulty, the braking signal is given with the left or right arm extended upward.
About pedestrians
As practice shows, pedestrians usually don't understand or misinterpret drivers' light and sound signals.
Therefore, if you intend to let a pedestrian pass at a crosswalk (which you are obliged to do) or just on the road — don't flash high beams, implying "go ahead, I'm letting you pass." It's much more effective and clearer to wave your hand along the "zebra."
A sketch from Germany
In Europe, motorists also use the language of signs. But a number of strict restrictions, for example on sound signals, somewhat impoverish this language. Here are a few examples of conventional signals from drivers in Germany.
Hazard lights are allowed only for their direct purpose — if the car broke down. One of the few exceptions: if there's a traffic jam on the autobahn, the car at the tail of the jam can turn on hazard lights so that cars approaching it (speeds on autobahns are around 160–200 km/h) have the opportunity to brake in advance.
Truck drivers thank each other by turning on the right and then the left turn signal (this signal is also used in our country).
Flashing high beams is prohibited, but the international danger signal (2 flashes of high beam) is sometimes used.
Sound signals are also prohibited. The horn can only be used to prevent an accident, otherwise a fine may be issued.
And a few more gestures
While preparing the material, I came across interesting information that I can't help but share. These are gestures made by minibus drivers to potential passengers:

and gestures for coordinating the driver's actions during maneuvering:

So
The language of conventional signals among motorists is quite multifaceted. It allows both warning of danger, asking to let pass, thanking for a service, and expressing displeasure with a neighbor on the road.
It remains only to once again call for politeness on the roads, so that grateful flashing of hazard lights is used more often than displeased honks.








