The investigation of car-following behavior of test drivers on Bangkoks roadways is presented in this research. The experiment was carried out on surface streets and freeways in Bangkok with actual traffic conditions using five passenger cars equipped with global positioning system devices. Traffic flow characteristic was classified into congested and uncongested conditions, resulting in four regimes of driving characteristics on the two roadway types (i.e. uncongested surface street, congested surface street, uncongested freeway, and congested freeway conditions). The experimental GPS data consisting of distance, speed, and acceleration of the vehicles at every 0.1 sec were employed for the calibration of the 1st and the 5th GM models in order to determine various driving behaviors in different traffic and roadway conditions. The consequences of the analyses of fundamental car-following parameters indicated that the drivers maintained very close separation distance at very high speed ranges under uncongested freeway condition, which absolutely violated the safe-distance concept. The change in individual speed patterns influenced on the follow drivers behavior more remarkably than other factors. Further, the increasing speed disturbance indicating more aggressive driving was found from the last two drivers of platoon. The results of model calibration showed that the drivers would have lower sensitivity under uncongested conditions and higher in congested conditions. Moreover, they would have faster reaction when drove in congested conditions. The model calculation gave the R2 between 0.4-0.8 and 0.4-0.9 for the 1st and the 5th GM models respectively, indicating that the 5th GM model did not contribute much improvement to another. The evaluation of the models at microscopic level revealed that the predicted speed values from both models agreed well with the measured speed data, meanwhile more obvious deviation between predicted and measured traffic volumes was exhibited for macroscopic evaluation. Finally, the comparison of car-following characteristics from a few researches conducted in USA (GM and LSU) as well as Japan (HU) vs. those from this study (so called CU) concluded that the GMs original car-following data were close to the CU results for both uncongested surface street and freeway conditions. However, it was evident that CU car-following characteristics were remarkably different from the others for congested conditions.
