The California Bearing Ratio (CBR) method is a widely used method for designing flexible pavements. It is considered to be more accurate because it takes into account the characteristics of the soils, which play a crucial role in the performance and durability of flexible pavements. The other options mentioned, such as traffic densities and specification of road making materials, are also important factors in pavement design, but they are not directly related to the accuracy of the CBR method.

Characteristics of Soils:
One of the key factors in designing flexible pavements is understanding the properties and behavior of the underlying soils. The CBR method takes into consideration the soil's bearing capacity, which is a measure of its ability to support loads without excessive deformation. The CBR value is determined through laboratory testing and represents the ratio of the force required to penetrate a soil sample with a standard piston to the force required to penetrate a standard material (usually crushed stone) with the same piston. This value provides an indication of the soil's strength and stiffness, which are important for designing the thickness of the pavement layers.

Importance of Soil Characteristics:
The characteristics of soils, such as their composition, moisture content, compaction, and shear strength, have a significant impact on the performance of flexible pavements. Soils with higher CBR values are generally stronger and more resistant to deformation, which means they require less thickness of pavement layers. On the other hand, soils with lower CBR values are weaker and more prone to deformation, requiring thicker pavement layers to distribute the loads and prevent excessive settlement or rutting.

Role of CBR in Pavement Design:
The CBR method uses the CBR value of the subgrade soil to determine the required thickness of the pavement layers. By considering the CBR value along with other factors such as traffic loads and climatic conditions, engineers can design a pavement structure that can withstand the anticipated stresses and deformations. This ensures that the pavement will perform well and have an adequate service life.

Conclusion:
In conclusion, the California Bearing Ratio method of designing flexible pavements is considered to be more accurate because it takes into account the characteristics of the soils, which have a significant impact on the pavement's performance and durability. While traffic densities and specifications of road making materials are important considerations in pavement design, they are not directly related to the accuracy of the CBR method.