This Diploma Thesis aims to investigate the impact of mobile phone use to traffic speed. In order to achieve this objective, an experimental process in real road conditions was carried out. Linear and lognormal regression methods were used to investigate the influence of mobile phone use and certain other parameters in the variable examined. These parameters concern the vehicle headways and certain driver characteristics, like gender and annual mileage driven. The mathematic models were developed, concerning the total mean speed, the mean speed of free and interrupted flow. From the models application it appears that the mobile phone use leads to statistically significant decrease of traffic speed. Furthermore, men and women drivers reduce the traffic speed similarly when they are using mobile phone.