EVALUATION OF ROAD TRAFFIC SAFETY LEVEL IN THE STATE MAIN ROAD NETWORK OF LATVIA

Evaluation of road traffic safety level may be done using several methods. The methods of accident rate and accident frequency used in this paper provide an opportunity to analyze dangerous road sections on all state main roads of Latvia. Latvia has 15 state main roads, they lead through 24 out of 26 districts, and their total length in Latvia is 1740.8 km. Analysis of road traffic accident statistics was carried out basing on the data available at Road Traffic Safety Directorate for the time period of three years (2005–2007). Critical value of accident rate has been calculated for state main roads network and its numerical value is 1.81 accidents per 106 vehicle km. Besides, relation between traffic intensity and accident rate for state main road network of Latvia has been achieved as well.


Introduction
Widely in the world to analyse and estimate an accident situation in civil engineering many different methodologies and methods are used (Giretti et al. 2009;Hoła 2009;Zavadskas, Vaidogas 2008;. Different methods for the evaluation of sustainable safety (Vaidogas, Juocevičius 2008) and road traffic safety level may be used to determine dangerous sections on roads (Kapski 2006;Kapski et al. 2007;Lama et al. 2006). One of the most frequently used criteria for safety evaluation not only on roads is accident frequency (AF) and accident rate (AR) (Hoła 2007;Sokolovskij 2007;Šliupas 2009).
The analysis done by authors covers the state main road network in Latvia. The function of the state main roads is to provide connections with foreign countries and capital cities of foreign countries. Latvia has 15 state main roads, they lead through 24 out of 26 districts, and their total length in Latvia is 1740.8 km.
Analysis of road traffic accident statistics was carried out basing on the data available at Road Traffic Safety Directorate for the period of three years (2005)(2006)(2007).

Accident frequency
One of the most frequently used analytical methods for determining the road traffic safety level is the calculation of AF. This value was determined for every km of state main roads. (1) where AF -accident frequency, accident/km); Accnumber of road traffic accidents per 3 years; L -length of analysed road section, 1 km; T -reviewed time period, 3 years.
Usually road sections with similar technical parameters are chosen and average frequency of accidents (AF ave ) is calculated for each road section: (2) where AF ave -average frequency of accidents, accidents/km; AF i -total number of AF in specific section, accidents/km; n -number of sections in general group.
According to PIARC Road Safety Manual the accident frequency limit value is determined which will be regarded as the min dangerous accident frequency AF lim : . ( After determining the AF for all sections it is compared with AF lim . With this approach the most dangerous road sections according to AF are determined.

Accident rate
The AR was determined that characterised the risks to which road users are subjected in a certain road section. The AR was calculated for each road section, as well.

EVALUATION OF ROAD TRAFFIC SAFETY LEVEL IN THE STATE MAIN ROAD NETWORK OF LATVIA
Ziedonis Lazda 1 , Juris Smirnovs 2 where AR -accident rate, accidents/vehicle km × 10 6 ; Acc -number of road accidents per 3 years; L -length of reviewed section, 1 km; T -reviewed time period, 3 years; N -annual average daily traffic (AADT) vpd. The formula given in the PIARC Road Safety Manual is used to determine the limit value of AR; if this value is exceeded it may be stated that the analysed road section is dangerous to traffic: where AR crit -critical value of AR, accidents/vehicle km × 10 6 ; AR ave -average value of AR in specific road network, accidents/vehicle km × 10 6 ; T -reviewed time period, 3 years; L -length of reviewed section, 1 km; N -AADT, vpd (for the state main roads AADT = 5305 vpd); C -statistical constant with 95% level of confidence (C = 1.645).

Accident frequency and critical accident rate
Basing on the formula given before Eq (3) and Eq (5) AF lim and AR have been found. Table 1 shows the data on average value of AR ave and AF ave for each state main road. AR crit indicates the limit value of AR which was calculated with respect to the whole state main road network. AF lim indicates the critical value of accident frequency for the whole state main road network.
Relation between traffic volume and AR in the state main road network (Fig. 1) may be expressed as follows: Considering the AF, the road with the worst properties (AF = 5.74) is the road A4 Rīga bypass (Baltezers-Saulkalne), however, considering the AR, the road with the worst properties (AR = 1.96) is the state road A12 Jēkabpils-Rēzekne-Ludza-Russian border (Terehova).

Practical use of accident frequency and accident rate
According to the formulas reviewed above, the AF and the AR was determined for every km of state main roads. As an example the calculations of one state main road A4 Rīga bypass (Baltezers-Saulkalne) may be reviewed.
Characteristics of the existing roads: Road A4 is located in Rīga district. Total road length is 20.4 km. Max permitted driving speed outside urban areas is 90 km/h, in urban areas 70 km/h and 50 km/h. In 2005 the AADT on road A4 is shown in Fig. 3.
Analysis of statistical material was done base on the data available at Road Traffic Safety Directorate for three years (2005)(2006)(2007). 380 accidents happened in the reviewed time period: 82 were heavy road accidents, 15 persons were killed and 140 injured.
In the Table 2 the values of AR are given which were determined according to Eq (4). To determine which road sections are dangerous for traffic the AR crit was calculated for the whole network of state main roads according to Eq (5) and AR crit = 1.18. Fig. 4 shows those road sections on state main road A4 where the values of accident factor exceed critical limit values, therefore these road sections may be regarded as dangerous for traffic.

Conclusions
The values of AR crit and AF lim calculated in this paper provide an opportunity to identify dangerous road sections. Calculated values of AR and AF provide an opportunity to define priorities for the needs to reconstruct dangerous road sections in the state main road network of Latvia.