The international roughness index (IRI) is one of the important criterion used to determine the roughness of roads. Various devices are available to determine IRI, however, they are mostly used to determine the condition of highways. Studies to determine the roughness of nonmotorized roads, such as bicycle roads, which have become increasingly important in recent years, are limited. Therefore, this study presents a cost-effective roughness measurement method that can measure both highways and nonmotorized roads. For this, a mountain bike was used as the test vehicle, and for the first time, the stiffness and damping coefficients of the bicycle tire were taken into account when calculating the IRI. First, the stiffness and damping coefficients of the bicycle's front tire were calculated. These coefficients were then used as input data for the equation of motion with one degree of freedom, and the IRI was calculated. The 660-m long divided highway (1,320 m in total) on the Bursa Uludagg University campus was chosen as the test road. A laser profilometer device was used to quantify the reference IRI values of this test road. The same road was then measured by bicycle. The obtained results were evaluated using mean absolute percent error, mean absolute error, root mean square error, and mean square error metrics. All these indicators showed very promising results, suggesting that this new method could be a cost-effective and reliable approach to determining the IRI of highways, especially nonmotorized roads.