INTERACTIONS BETWEEN ACCIDENT RATE AND TRAFFIC VOLUME

Transcription

1 INTERACTIONS BETWEEN ACCIDENT RATE AND TRAFFIC VOLUME Algis Pakalnis, Deputy Director, Mindaugas Dimaitis, Head of Road Division, Andrius Gužys, Road Division Group Leader, Transport and Road Research Institute, Lithuania Introduction Transport is the main part of modern national economy. However, there always exist contradictions between the society, which needs large mobility, and public opinion when continuous delays, poor quality of transport services and road accidents become more and more unacceptable. Transport needs are continuously increasing due to two factors public mobility and the growth of cargo transportation. In the field of passenger transport this determines a large demand of cars. In the last 30 years the number of cars in Western Europe has increased three times. And though in many EU Member States the number of cars became stable, in the candidate countries, including Lithuania, no stabilization takes place. If in the year 1992 there were 234,4 cars per 1000 Lithuanian inhabitants, in 2002 the number increased to 397,3. The increasing demand of cargo transportation is mainly determined by the changes in European economy and production system. In last 20 years the economy of warehouses has been transformed into the economy of consumption. Having eliminated the borders within European Community the production system called just in time or renewed reserves was created, which requires high volume of transportation. It is predicted, that until 2010 the traffic of heavy vehicles will increase by nearly 5, compared to For Lithuanian integration into a common economic space of EU, these changes are also inevitable. Due to the fact that important European transport corridors cross Lithuania, the growth of heavy transport is generated by the increasing economy and transit amounts. Therefore, the growth of heavy transport exceeds the total transport growth; the annual increase in heavy transport in 2002 amounted to 17%. Dynamics of traffic volume on Lithuanian roads With Lithuanian integration into West European economic space the national transport system has been rapidly changing. Dynamic changes in the number of vehicles and transportation needs cause the growth of traffic volume (Fig. 1). In last eight years the increase in the number of vehicles was 8,6 % per annum, in traffic volume - 5,6% per annum. 1

2 Number of vehicles Traffic volume Fig. 1. Change in the number of vehicles and traffic volume, In the period traffic volume on main and national Lithuanian roads had increased by 45%. The change in uniform traffic flows was significantly affected by the economic changes: with the developing level of economy traffic volume on our roads had also been increasing. However, the growth of traffic volume on different road sections was not the same, depending on the increasing number of cars, concentration of industrial and trade enterprises in the largest cities and change in international transit of heavy vehicles on the corridors of European road network. Fig. 2. Traffic volume on main and national Lithuanian roads in The comparison of light and heavy transport flows on main and national roads showed that the share of light transport made 88,8% of the total flow, whereas the share of heavy vehicles 11,2%. On Lithuanian roads part of European road network a similar distribution still remained: light transport made 86,9% of the total flow, the heavy transport - 13,1%. 2

3 Seasonal variations are characteristic to Lithuanian road network (Fig. 3). Seasonal irregularity depends on the size of residential areas and attraction centres, connected by the road. It was noticed, that the largest traffic volume is in August, the lowest in January. Such traffic distribution in summer time is caused by a holiday period, in wintertime bad meteorological conditions Months Seasonal coefficient <1,5 1,5-2,0 >2,0 Fig. 3. Standard average curves of annual change in traffic volume, When analysing the change in traffic volume on main and national Lithuanian roads by weekdays, it was determined that, in general, the most trafficked day is Friday (Fig. 4). However, there are road sections where due to the industrial-commercial activities the highest traffic volume is observed on another day of the week. For example, the roads around Marijampolė city, where the highest traffic volume is on Tuesday (Wednesday is a market-day of cars) Weekday Fig. 4. Weekly traffic distribution on main and national roads. 3

4 Active time of the day has a significant influence on traffic volume. Difference between the highest and the lowest hourly traffic volume is 9,6 times. When studying the change in traffic volume on main and national roads by hours it was observed, that the most intensive hour is 5 p.m. 1,800 1,600 1,400 1,200 1,000 0,800 0,600 0,400 0,200 0, Fig. 5. Hourly traffic distribution. Accident rate on Lithuanian roads The growing number of vehicles and traffic volume, besides a low culture of road users, enhances the risk of accidents. In 2002 there were 6141 injury accidents and fatalities in Lithuania, 697 people were killed and 7491 injured. On main and national roads (urban road sections were not included) 1607 injury accidents and fatalities took place, 377 people were killed, 2116 were injured Total number of accidents The number of accidents on main and national roads Fig. 6. Change in the number of injury accidents and fatalities in Analysis of change in the number of accidents shows, that the reducing number of injury accidents and fatalities since 1998 has started to increase again (Fig. 6). A similar situation 4

5 could be noticed on main and national roads (accident number does not include urban road sections). The above data shows that only one/fourth of the total number of injuries and fatalities takes place on the main and national roads. This significant distribution requires paying more attention to accident situation in cities and settlements and to traffic safety measures Killed Injured < 4 years 5-14 years years years years years years >64 years Fig. 7. Distribution of injured or killed people by different age group The increasing number of accidents in the last years (2001 and 2002) causes great anxiety. Statistics show: nearly half of road accidents involve pedestrians and bicyclists; one fifth - is caused by young drivers (when the driving experience is less than 1 year); one third is caused by pedestrians and bicyclists. The groups of young (15 24 years) and able-bodied (25 44 years) people are those who suffer in road accidents most of all. Having this statistical data and knowing that the country s economy lies on the shoulders of the youngest and most able-bodied inhabitants, it is an urgent task to search for effective methods and measures in reducing the number of accidents. Interaction between accident rate and traffic volume Accident rate is one of the main indicators reflecting the processes of transport system as well as their positive and negative development. When studying transport system all its elements are divided into 4 groups: road, vehicle, driver and road environment. Road environment is sometimes linked with the road. Meteorological conditions are described as features of road and road environment. Accident rate is a complex characteristic of road-vehicle-driver-road environment system, since the accident is caused or could be caused by all these factors, describing the system s operation. The summary study results show that on a macro-scale there is a direct relation between accident rate and traffic volume. The higher is traffic volume the larger ratio between the number of accidents and the length of road section (Fig. 8). This relationship is constant and approved by many countries. Though on the micro-scale, when examining specific high-accident locations, this relation is not so direct and monolithic. 5

6 35% 3 25% 2 15% 1 5% Traffic volume Accident number Accident density Veh./day Fig. 8. Relationship of traffic volume and accident rate on a macro-scale The entire features characteristic to road-vehicle-driver-road environment system could be divided into constant and variable. Constant features are: road geometry, road infrastructure, pavement condition and road environment. Variable features are as follows: traffic volume, vehicle technical condition, driver s state, and meteorological conditions. The fact is the feature constant or variable makes a great influence on the processes of transport system. Detailed analysis of high-accident locations shows that accident nature is closely related to road infrastructure (Fig. 9). Road infrastructure and road environment are attributed to constant features. Namely the combination of constant and variable features determines different nature and different density of accidents on the road. 6

7 according to road infrastructure High-accident location in residential area High-accident location in thinly populated area High-accident location on urban street (90km/h) High-accident location on main or nation. road, crossing the settlement (v leist <90km/h) High-accident location on the approaches to cities and settlements (v leist 90km/h) between various groups of road users between different groups of road users between the same group of road users between vehicle and pedestrian (bicyclist) between vehicles between vehicle and road environment elements Pedestrian s (bicyclist s) fault Driver s fault Caused by road or its environment Caused by meteorological conditions Fig. 9. Relation between accident and elements of transport system. Rather often in accident studies the effect of certain factor on road accidents is decided based on statistical data. For example, if with the growing traffic the number of pedestrian-involved accidents increases, it can be stated, that traffic volume is the accident-influencing factor. Though, in reality, the cause of accident is related to pedestrians, getting into the carriageway, while traffic volume is a factor that influences accident risk, i.e. the more vehicles travel on this road section the higher is probability for the pedestrian to be run over by a car. Thus, traffic volume as well as the other factors can possibly be the cause of accident and the factor influencing accident risk. This differentiation of functions is important in studying the specific high-accident location as well as processing general accident data and interpreting the results. In accident studies traffic volume is assessed as the accident-influencing factor. As mentioned earlier, it belongs to the group of variable features; therefore, it cannot be treated as constant and is 7

8 the function of time and place. Moreover, traffic volume, if compared to other factors, has a different effect on accidents. In this aspect accidents could be divided into two groups: accidents where traffic volume was the main factor; accidents where traffic volume had no effect or was not the main factor. The first case is characteristic to intensive city streets. On rural roads it is rather rare. It could be caused by traffic increase during mass events, transport flow on temporary by-passes, etc. In this case vehicle density reaches the limit when the moving freedom of individual driver becomes very restricted or not possible at all. The driver has no possibilities to choose an acceptable driving regime and to keep safe distance; the required attention and concentration is too large. In such situations accidents are treated as the consequence of a very large traffic volume. The second case is more often met in practise, where traffic volume is part of the entire complex of factors. In such case the relation between accidents and traffic volume is rather difficult to determine and additional information has to be used, for example, vehicle speed (Fig. 10). Vehicle speed also reflects the effect of entire complex of factors: traffic volume; speed restriction signs; road geometry and road environment; speed of the other road users; driving culture and habits; vehicle technical possibilities; financial resources. 4 35% 3 25% Cars Articulated heavy vehicles 2 15% 1 5% > 140 km/h Fig. 10. The speed of different-type vehicles on road A17 14,40km. 8

9 Driving speed reflects direct and indirect effect of traffic volume and other factors on driver s thinking and psychology. The study of driving speeds covers the effect of three main factors (i.e. road, road environment and traffic volume) on the driver. This is the important information for accident investigation, since the integration of speed, road infrastructure and traffic volume data provides a possibility to determine speed variations and, correspondingly, the potentially dangerous road sections. For example, lets take the articulated heavy vehicle (Fig. 10), moving at a given speed and the tractor, entering the main road from the exit. In this case traffic volume determines the probability of such situation in a certain time interval, also distance between vehicles and the probability for one of the drivers to avoid collision. As mentioned above, traffic volume is a variable feature of this system. It may vary with respect to time and place. Variation of traffic volume in time is an important aspect, which is often not taken into consideration in accident studies. When studying accident distribution in time and determining the law of traffic volume variation in time, it is very important to specify time interval. The law of traffic volume variation is closely related to time interval and time step in this interval (Fig ) Traffic volume 4 2 January February March April May June July August September October November December Fig. 11. Yearly variation of accidents and traffic volume 9

10 Traffic volume 4 2 Monday Tuesday Wednesday Thirsday Friday Saturday Sunday Fig. 12. Weekly variation of accidents and traffic volume Traffic volume Fig. 13. Daily variation of accidents and traffic volume The law of traffic volume distribution in time indicates a factual transport flow in a study period. When analyzing a different variation of traffic volume and accidents in time interval it is possible to identify the other accident causes, depending on length of time interval and time step. Thus, traffic volume and the law of its variation allows us to discuss the effect of the other factors influencing accident rate. When studying accidents in one-year interval (Fig. 11) a different nature of traffic volume and accident distribution laws reflects the increased effect of meteorological factors pavement slippery, visibility and the higher importance for the whole complex of accident factors. 10

11 Such difficult to describe and to measure factors as the drivers psychological condition could be observed and studied not directly but with the help of variation laws of other factors more easy to determine. The law of traffic volume variation in a week s time interval (Fig. 12) makes it possible to better recognize psychological condition of the driver as well as his attention concentration, depending on a day regime and the chosen route. A differing effect of psychological factors (concentration, tiredness) and also meteorological conditions (visibility) on accidents is obvious when studying the law of traffic volume variation in one-day time interval (Fig. 13). This different nature of the laws of traffic volume and accident rate often remains outside the studies of accident rate. The main task in the analysis of accidents is to determine accident-influencing factors and to prove their effect. Namely the process of proving a certain factor can reveal the real accident causes, however such proof is not always possible. This can lack statistical data as well as relations between the studied accidents and specific factor. A complex investigation of accident factors gives a possibility not only to determine the validity of earlier predicted factors, but also to define more precisely the effect of other factors or to take notice of new factors, the direct influence of which on the accidents is technically or statistically not possible to record. Traffic volume is one of the factors, which has a direct influence on the accidents, changes accident probability, and in the correlation with other factors makes the effect on accident nature and allows to determine additional factors not directly recorded. 11