Traffic Indices for the use of the Belgian motorway network

Transcription

1 TRANSPORT & MOBILITY LEUVEN TERVUURSEVEST 54 BUS LEUVEN BELGIUM TEL FAX WORKING PAPER NR Traffic Indices for the use of the Belgian motorway network Filip Vanhove, Transport & Mobility Leuven Griet De Ceuster, Transport & Mobility Leuven January 2003 Abstract The Belgian Federal Government developed the START/SITTER-system to collect and process the data provided by the single inductive loop detectors on the motorways. Transport & Mobility Leuven developed a number of traffic indices to monitor traffic flow, congestion and emission costs. At the moment, these indices are implemented into the system of the government. Key words congestion, indices, motorways

2 TRANSPORT & MOBILITY LEUVEN 2 1. Introduction Congestion is a widespread phenomenon on the Belgian motorway network. The movement of people and goods is heavily affected. The effects manifest themselves in increased travel times, increased fuel consumption, loss of productivity and environmental damage. Congestion and mobility issues have been discussed and debated for a long time. This research attempts to provide some information about some of those issues in ways that both the public and professional groups can understand. Specifically, this study aims to monitor motorway congestion in Belgium. The history of the traffic data system in Belgium The Belgian Federal Public Service Mobility and Transport collects traffic data for all the Belgian motorways. These numbers are collected using single inductive loop detectors, which are located before and after almost every group of on and off ramps. Data is collected for each traffic lane separately, sometimes also on the on and off ramps. Every minute, data on speed, volume and traffic density is transmitted. On some locations cameras are used instead of inductive loops, but their output is limited to the output that is generated by the single loop detectors. In total, measurements are made at about 1200 places. This traffic data is processed in real time in the START/SITTER system. In this system, implemented by Siemens, the data is collected, processed and stored. The information is accessible through a web-based interface for authorised parties. At the end of 2001 it became obvious that processing of this wealth of information could contribute a lot to the mobility policy. Transport & Mobility Leuven designed a system of indices to map the mobility and the generated congestion and environmental costs. The theoretical design of these indices was completed in April 2002, after which the implementation in the START/SITTER system could be started. Transport & Mobility Leuven was also involved in the implementation as scientific advisor. Validation of the results was performed and advice was given when problems occurred. The most significant problem was the reliability of the traffic data. Incorrect data has to be filtered out and missing data has to be coped with. This paper consists of two parts. Firstly, an overview will be given of the pre-processing of the data that had to be done before the indices could be calculated. In the second part of the paper, the indices themselves will be discussed.

3 TRANSPORT & MOBILITY LEUVEN 3 2. Pre-processing the data Completion of missing data Every minute, mean values for the last minute are received by the central computer from the detectors, which starts by testing the validity of the incoming data. For each parameter, the measurement is compared with predefined intervals. These interval definitions are based on physical laws and common sense. The number of vehicles and their speed can t be negative, for example, and the mean speed can safely be assumed to be lower than 200km/h. When a measurement doesn t fall into these boundaries, the value is rejected. As a result only valid and missing data points are left. A number of processing steps need complete data sets. Therefore, the missing data has to be completed. A layered data completion process was established, starting with the completion of data on minute level and ending with the completion on the basis of hourly data. Firstly, missing data for one or two minutes is completed on the basis of linear regression. Then, gaps up to twelve consecutive minute values are completed using the same technique, treating the completed one and two minute gaps as valid data. The linear regression uses as many data points before and after the missing data, as there are missing data points. Further completion of the data takes place after it has been aggregated from data per minute to data per hour. First data of neighbouring places is used. Neighbouring places are detectors up or down stream, without any motorway access points or exits in between. Then final completion is performed on the basis of mean values of similar days. Normalisation of speed values It was observed that the measured speed values are often unrealistic. The cause of this is the measurement system, consisting of single inductive loops, and of the measurement algorithms. Both these causes are the responsibility of other governmental entities, and as such have to be seen as external factors. To alleviate this problem, the measured speed values are normalised. This means that the speed values are rescaled in such a way that the mean speed for normal hours of the day becomes 100 km/h. Normal hours are those with a sufficient amount of traffic and free-flow traffic conditions. Correction fraction of trucks Not only do the measurement system and algorithms produce incorrect speed values, they also yield incorrect fractions of passenger cars and trucks. This is caused by the use of single inductive loop detectors. Speeds aren t measured directly by these detectors, but are calculated. In addition, the used algorithm frequently misinterprets slow passenger cars as trucks when traffic-jams occur. As a result, the fraction of trucks is much too high. The correction procedure consists of three steps. Firstly, all trucks on the leftmost driving lane are discarded, if the road has more than two lanes. Then, all data is aggregated per detector place (one value per place instead of one value for each lane of that place). Finally, the number of trucks is limited to a floating maximum. This floating maximum is calculated based on exponentially weighted moving average of the measured volume of trucks, limited to 1.75 times the median of the number of trucks per five minutes.

4 TRANSPORT & MOBILITY LEUVEN Speed Trucks measured Trucks corrected 200 Trucks/5 minutes :00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 Figure 1 Example of the correction of the fraction of trucks. The measured numbers of trucks are indicated in pink, the numbers after correction are indicated in turquoise. Detection of traffic-jams Several traffic indices give information about traffic-jams. Therefore, a traffic-jam detection step precedes the calculation of the indices themselves. In this step, information about the traffic-jams is stored in the database for future use in calculation of the indices. The detection algorithm was developed on the basis of the fundamental speed-flow diagram, as illustrated on Figure 2 and Figure 3. Every minute, the following algorithm is walked through for every detector. A traffic-jam starts when the normalised speed drops below 50 km/h. A traffic-jam ends when the intensity drops below 25 vehicles per minute and per lane, AND the normalised speed exceeds 75 km/h. In a second step, traffic-jams that are too short, or too short periods between two successive traffic-jams are removed. Two traffic-jams with periods of less than 10 minutes between them are merged into one, traffic-jams of less than 10 minutes are discarded.

5 TRANSPORT & MOBILITY LEUVEN START OF FILE TRAFFIC-JAM Figure 2 A traffic-jam starts when the normalised speed drops below 50 km/h; the speeds on the figure are not yet normalised END EINDE OF FILE TRAFFIC-JAM Figure 3 A traffic-jam ends when the intensity drops below 25 vehicles per minute and per lane, AND the normalised speed exceeds 75 km/h; the speeds on the figure are not yet normalised. The results of the algorithm are satisfying. One remaining problem is the identification of the type of traffic-jam. To be able to properly distinguish between recurring traffic-jams and incidental traffic-jams, external information is needed, for example, data collected by the federal police. Such data is currently not available, therefore the distinction between the two types of traffic-jams is only made on the basis of the time of day.

6 TRANSPORT & MOBILITY LEUVEN 6 Traffic indices - General remarks The aim of this project is to produce monthly overviews of the congestion on Belgian motorways, similar to consumer price indexes. The data that is stored in databases is processed, resulting in tables and diagrams of traffic flow, congestion, emissions and external costs. The aim is to develop an automated process to transform the raw data into tables and diagrams that can be easily interpreted. An accurate overview of the actual situation is essential for well-considered policy making. These traffic indices deliver an objective and scientific framework, based on up-to-date information. Selection of days Each index is calculated for several selections of days. Firstly, a number is derived taking into account ALL days in the given period. The next two values are for all days, except special days; all weekdays except special days and all weekend days except special days. Special days are defined as public holidays, school holidays, special events, Finally, values are calculated for each day of the week (Monday, Tuesday, ) without special days. Regional and local traffic indices Two types of traffic indices were developed: regional and local traffic indices. A regional traffic index provides a general overview of the traffic throughput. The regional traffic index is a macroscopic index which is not able to visualise local bottlenecks. By means of this index regions can be compared to each other and time series can be constructed. Local traffic indices describe for each detector place the evolution of traffic throughput. They provide a good picture of local bottlenecks. These indices can be visualised on a map by varying the colours and thickness of the lines. Selection of regions The regional traffic indices are calculated for ten different selections. Five selections consist of regions around big cities (Brussels, Antwerp, Gent, Charleroi and Liege). Then there are three larger regions: Flanders, Wallonia and all detectors of Belgium. The last two selections consist of traffic coming into Belgium and leaving the country. Road sections All traffic indices are calculated on the basis of road sections. Each road section of approximately five kilometres has one set of detectors one detector per lane. Freight traffic Freight traffic has an important impact on mobility and environment. Therefore, most of the indices are calculated twice: once for all traffic (passenger cars and trucks) and once only taking the trucks into account. This way, the contribution of the heavy vehicles can be estimated. It must be noted that the way trucks are taken into account can vary. Some indices don t distinguish between cars and trucks. These indices use vehicles as unit. Other indices use passenger car equivalents. In this case, trucks are considered as two passenger cars.

7 TRANSPORT & MOBILITY LEUVEN 7 3. Regional Traffic Indices In this section, an overview will be given of the indices that are calculated for each region. Vehicle kilometres This value monitors the total amount of kilometres that are covered in a certain region. For each road section the daily average volume is multiplied by the length of the road section. The sum of these values for the considered region, delivers the final result. In this calculation, no distinction is made between passenger cars and trucks. In Figure 4, a draft calculation of the vehicle kilometres in Belgium is given. It is obvious that traffic has increased every year. Another point of notice is that the values for January are significantly lower than the values for the rest of the year. 85 Vehicle kilometres in Belgium (working days) 10 6 vkm/day Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 4 Draft calculation of vehicle kilometres in Belgium Regional volume index The regional volume index gives the average traffic volume on a lane of the motorway network. Firstly, the daily average volume is multiplied by the length of the associated road section, but this time, passenger car equivalents are used. The sum of these values for the region considered is divided by the total length of the motorways in the region. Regional lane index One way of looking at roadway congestion is to estimate the amount of travel that occurs in congested conditions. The percentage of the motorway travel occurring on sections of roadway with congested travel conditions is a way of determining the amount of motorway system congestion. A lane-kilometre of freeway that has vehicles per day is considered to be congested during peak periods.

8 TRANSPORT & MOBILITY LEUVEN 8 Regional lane index for Belgium (working days) 55% 50% 45% 40% 35% % Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 5 Draft calculation of regional lane index for Belgium Regional inconvenience index This index estimates congestion levels as perceived by individuals. The regional inconvenience index gives the percentage of the vehicle kilometres that are covered on congested road sections. The calculation is similar to the regional lane index, only this time vehicle kilometres are considered instead of road lengths. Congested road sections are also defined as those with a daily average volume higher than Arithmetically, the percentage of congested travel (inconvenience index) will always be greater than the percentage of congested motorway (lane index). Regional speed index The regional speed index gives the average speed on the motorways in the regions considered. It is calculated on the basis of a weighted average, taking into account the length of the road sections and the number of vehicles on them. For each road section, the average speed is multiplied by the total traffic volume on the road section for the period considered, and by the length of the road section. The sum of these values for all road sections of the region considered is divided by the vehicle kilometres. The calculation is made separately for passenger cars and trucks. This indicator is interesting, because it is a way of measuring the efficiency of a motorway system. The number of hours of travel delay in a region Travel delay is the most apparent impact of congestion to the drivers. Two types are seen: recurring delay occurs when travel times are longer during peak hours. Accidents, breakdowns and other events that temporarily decrease roadway capacity, cause incident delay. Each time the normalised free flow speed hasn t been reached, time is lost. For each hour during which the average speed is lower than 90% of the free flow speed, the difference is calculated between the measured travel time (which is calculated as the inverse of the measured speed) and the free flow travel time. Then this difference is multiplied by the traffic volume during the hour considered, and the length of the road section. The sum of these values for all road sections of the region considered results in the number of lost hours. The calculation is made separately for passenger cars and trucks, and for three parts of the day: morning peak, evening peak and rest of the day.

9 TRANSPORT & MOBILITY LEUVEN 9 The estimates of the delays provide additional insights into the congestion level, which is not always evident with the other indices. When the regional volume index increases, recurring delays are measured. In this index, incident delay is also taken into account. Incident delay is not caused by high traffic volumes, and incident congestion may be a much greater percentage of total delays in less congested areas. Congestion costs Another method of assessing impact is to look at the monetary value of travel delay. First the number of lost hours is calculated for each road section. In this case, the lost hours are added up for each hour of the day separately. This is necessary because the value of lost time is different for each hour of the day, because travel purposes differ. The congestion costs are simply found multiplying the number of lost hours with the value of lost time for each hour of the day. The calculation is made separately for passenger cars and trucks. Three types of numbers are calculated: the total congestions costs, the congestion costs during structural traffic-jams and the congestion costs during incidental trafficjams. The weight of the traffic-jams Traffic-jam gravity is calculated as the multiplication of duration and length. The units that are used are kilometre hours. For each of the road sections in the region considered, the total duration of the traffic-jams during the period considered is determined. Then this total duration is multiplied by the length of the road section. Finally, the sum of these values for all road sections in the region considered is calculated. Travel delays and traffic-jams are closely connected. For this reason, the ratio of the number of hours of travel delay to the weight of traffic-jams is fairly constant Weight of traffic-jams (working days) kilometre hours Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 6 Draft calculation of weight of traffic-jams in Belgium

10 TRANSPORT & MOBILITY LEUVEN Local Traffic Indices The number of congestion days on a road section Using the traffic-jam data that is collected during the pre-processing stages, the number of congestion days can be determined for each road section in the region considered. The traffic-jam probability on a road section The traffic-jam probability indicates the probability of encountering a traffic-jam on the road section considered during a certain hour of the day. The length of recurring traffic-jams on a road section Using the traffic-jam data that is collected during the pre-processing stages, the length of recurring trafficjams can be determined for each road section in the region considered. Mean speed in a traffic-jam Using the traffic-jam data that is collected during the pre-processing stages, the mean speed in a trafficjam can be determined for each road section in the region considered. Mean volume in a traffic-jam Using the traffic-jam data that is collected during the pre-processing stages, the mean volume in a trafficjam can be determined for each road section in the region considered. 5. Environmental Indices Emissions CO, NO x, VOC, PM 10 The emissions are determined on the basis of emission factors as a function of speed. These emission factors were calculated on the basis of COPERT III, and the vehicle fleet of Belgium. For each hour of the day, the total emissions are found multiplying these emission factors (calculated for the speed of the hour considered) with the vehicle kilometres covered during this hour. Emission costs The emission costs are determined on the basis of the marginal external costs as determined by Mayeres [I. Mayeres, S. Ochelen & S. Proost (1996), The Marginal External Costs of Transport, Transportation Research D, 1 (2)]. Similar to the emissions, the emission cost are determined for each hour of the day, multiplying the pollution costs (calculated for the speed of the hour considered) with the vehicle kilometres covered during this hour. 6. Some conclusions Traffic flow on the Belgian motorway network has been monitored based on the automatic processing of the data provided by single inductive loop detectors. During 2002 and 2003, the results were further analysed. Each of the following areas has been examined by several indices: traffic volumes, traffic speed and emissions. The presented results are very valuable for further research on road accessibility, reliability of the traffic system, incident management, costs of congestion and pollution to society,