What primarily determines the effectiveness of a vehicle's braking system at low speeds?

The effectiveness of a vehicle's braking system at low speeds is primarily determined by the adjustment of the brake shoes. Properly adjusted brake shoes ensure that the brake pads or linings make optimal contact with the braking surface, enhancing stopping power and responsiveness. When the brake shoes are correctly positioned, they can either engage more effectively with the drum or disc, or release completely when not in use, which is vital for efficient braking, especially at lower speeds.

In contrast, factors like tire size, engine type, and brake fluid quality influence braking performance but are less critical specifically at low speeds. Tire size affects traction and stability, engine type may influence overall vehicle dynamics, and brake fluid quality is essential for the hydraulic system's functionality but does not directly relate to the immediate effectiveness of the brakes at low speeds. Therefore, the adjustment of the brake shoes is crucial for ensuring that the braking system performs optimally in this speed range.