Seasonal variation coupled with heavy axle loading is the key factor in rapid road deterioration in Pakistan. The serviceability loss is further accelerated by the fact that truck drivers and owners consider overloading as a profitable practice unaware of the adverse effects of this practice. Weigh-in-motion data from two stations located between two major cities of Pakistan (Peshawar and Rawalpindi) on Grand Trunk Road (N-5) were collected and analyzed. Analysis of variance and comparison of actual and designed truck factor were performed to identify the most damaging axle truck type. It was found that axle truck type 3 (single/tandem axle) is most damaging among all truck types. The actual truck factor for axle truck type 3 is 6.4 times greater than design truck factor. Regression expressions of different forms were also investigated to determine the relationship between truck factor and gross vehicular weight for the specified truck types. An optimum generalization strategy was used to prevent over-generalization and ensure accuracy. For data analysis, 75% of data was used to develop regression models and remaining 25% was to validate those models. The results show that the polynomial expressions performed best and provide a robust relationship that can be employed by the highway authorities to estimate truck factor from gross vehicular weight with a high degree of confidence. It was also observed that damaging effect of various types of trucks was very severe and quite high.