Clause 10.3

When changing lanes, the driver must give way to vehicles moving in the same direction in the lane into which he intends to move.

Brief application: a car changing lanes from the right lane into your lane is required to "give way" to you as a vehicle already moving in this lane without changing direction.

Clause 12.3

If a danger to traffic or an obstacle arises, which the driver is objectively able to detect, he must immediately take measures to reduce speed up to stopping the vehicle or safely bypassing the obstacle for other road users.

Brief application: when another car has actually already started entering your lane, there is a danger of collision, so you must choose a safe action — first of all, reduce speed (if necessary — up to stopping), rather than "insisting on your right of way."

Clause 10.1

Before starting to move, changing lanes, turning or making a U-turn, stopping or parking, the driver must give the appropriate signal and make sure that it will be safe and will not create obstacles or danger to other road users.

Brief application: this clause describes the general principle of maneuver safety; in this situation, it emphasizes that the other driver's maneuver should not create danger, and for you — it is a reason to anticipate risk and act cautiously in advance (reduce speed).

Clause 1.10 (term "Give way (not to create an obstacle)")

"Give way (not to create an obstacle)" — a requirement for a road user not to continue or resume movement, not to perform any maneuvers, if this may force other road users who have priority to change their direction or speed.

Brief application: this wording explains that "priority" in the lane does not cancel your need to react so that no emergency avoidance is required; the most typical safe action is to reduce speed and allow the other driver to complete the lane change.

Clause 1.5

The actions or inaction of road users and other persons must not create danger or obstacles to traffic, threaten the life or health of citizens, or cause material damage.

Brief application: even if the other driver is making a mistake, your actions should be aimed at preventing danger; reducing speed and "letting pass" is a practical way to fulfill this requirement.

Clause 13.1

The driver, depending on the speed, road conditions, characteristics of the cargo being transported, and the condition of the vehicle, must maintain a safe distance and a safe interval.

Brief application: when a car is changing into your lane, to maintain a safe distance/interval, it is logical to reduce speed and allow it to complete the maneuver without a trajectory conflict.

That is, the correct answer is "Reduce speed and allow him to complete the lane change," given that according to the definition of the Traffic Rules, the driver in case of danger is obliged to immediately take measures to reduce speed (clause 12.3), and the actions of road users must not create danger (clause 1.5) and must ensure a safe distance and interval (clause 13.1), even if the obligation to "give way" when changing lanes lies with the one who is changing lanes (clause 10.3).

When you are driving in your lane and a vehicle from the lane to your right has already started changing lanes into yours, the situation becomes a matter of practical safety. Formally, the driver who is changing lanes must perform the maneuver in such a way as not to create danger or obstacles for other road users. But as soon as you see that the lane change has actually begun and the trajectories may intersect, your main task as a driver is not to “prove you are right,” but to prevent an accident.

In such conditions, the most predictable and safest action on your part is to reduce your speed. This immediately increases your time and space buffer: the distance to the vehicle entering your lane grows, and the risk of collision decreases in case the other driver misjudges the distance or makes the maneuver more abruptly than you expect. At the same time, it is important to avoid sudden steering maneuvers, as a sharp “swerve” to the left or right can create danger for other vehicles in adjacent lanes.

Imagine a typical situation in dense traffic: a vehicle on your right turns on its indicator and begins to move into your lane, with only a small gap between you. If you continue at the same speed or, even more so, accelerate, you will effectively “close” the gap and increase the likelihood of a side collision. Instead, smoothly reducing your speed gives the other driver the opportunity to complete the maneuver, while you maintain control and stability, without forcing other road users to brake sharply or swerve.

This approach corresponds to the general requirements of the Traffic Rules regarding safe speed and the driver’s obligation to act in a way that does not create danger, as well as the practical principle that in a conflict situation on the road, the priority is to prevent an accident, even if the other driver is not maneuvering well.

Therefore, the correct answer is "Reduce your speed and allow them to complete the lane change," as this is the safest and most predictable action, which increases distance and reaction time and helps avoid a collision during an already started lane change.