Traffic accidents and road fatalities present a serious problem of the modern age. Statistics show that large percentage of traffic accidents with fatalities occur on horizontal curves which indicates that, in addition to drivers’ errors, causes of road accidents can be found in the characteristics of the road alignment. Driving safety depends on several factors and one of the main causes of accident occurrence is the lack of geometric design consistency in terms
of maintaining the desired travel speed. Design consistency refers to the ability of geometric characteristics of the road to conform to driver expectancy. A consistent road design ensures coordinated successive elements producing harmonized driver behaviour with no surprising events. Beside the successive elements consistency, a good road design must establish the balance of superelevation and side friction values in curves with actual driving speeds. Although numerous speed studies have been conducted, most of them were based on spot speeds and certain assumptions providing incomplete information about the actual speeds. The limitations of the existing studies indicate the need for more detailed research of actual driver behaviour and the inclusion of real speeds in the road design procedure. This paper presents the free flow speed analysis, with speeds recorded on a segment of the state road D1 in Dalmatia region of Croatia using an innovative GPS based methodology. This methodology allows the continuous speed data collection and gives an accurate picture of drivers’ behaviour. The data were collected on 27 km long road segment and used for three types of analysis: relationship between project speeds and actual speeds of most drivers, comparison between side friction factors (demand, supply and design) and creation of the operating
speed model in horizontal curves.

GPS; operating speed; project speed; horizontal curve; side friction factor; operating speed model