DESIGN OF CONNECTOR ROADS IN FOUR-WAY INTERCHANGES REGARDING DRIVING BEHAVIOR AND ROAD SAFETY

Transcription

1 DESIGN OF CONNECTOR ROADS IN FOUR-WAY INTERCHANGES REGARDING DRIVING BEHAVIOR AND ROAD SAFETY Matthias Zimmermann Karlsruhe Institute of Technology (KIT) Institute of Highway and Railroad Engineering (ISE) Otto-Ammann-Platz, Karlsruhe, Germany Tel: + 0-; Fax: + 0-; Matthias.Zimmermann@kit.edu Word count:,00words text + tables/figures x 0 words (each) =,00words Submission Date: //0

2 Zimmermann 0 ABSTRACT This report is based on a research project () served to examine the uses of group-i connector roads according to the German Guidelines for the Design of Motorways (RAA), funded by the German FederalMinistry of Transport and Digital Infrastructure (BMVI) and the Federal Highway Research Institute (BASt). The empirical studies are based on more than 0 connector roads. Initially, extensive investigations were carried out about the limits of the use of specific connector road cross sections. Even at very high traffic volumes no interferences could be detected on the connector road when the connector roads were combined with enough highcapacity exits and entrances. Therefore the future cross section of connector roads should be chosen only dependent on the number of lanes of the entrance and exit to the connector road. The analysis of the driving behavior and the accidents resulted in the finding that there were no advantages of connector roads with a two-lane cross sections Q based only on the length of the connector road at low traffic volume - it can even be of disadvantage for the traffic safety, especially with the entrance type E (the left lane is tapered before the entrance). Peculiarities of the adapted loop connector roads as well as direct connector roads with adverse relations of radii and/or long straight sections in between indicate that the absence of any definitions of relations of radii and lengths of straight sections, which the RAA () so far contain only for road stretches, should be reconsidered. The new findings in this regard were represented in two diagrams and are recommended for inclusion in the RAA.

3 Zimmermann TASK DEFINITION This investigation served the purpose of examining the uses of group I connector roads in the light of interdependencies of traffic volume and speed as well as traffic safety and is also looking into alignment-aspects. Figure shows the definition of the connector road groups for four- and three-way interchanges (group I) plus motorway junctions (group II). Figure : Slip road types and connector road groups with recommended radius speeds (VRampe [km/h])(raa, 00)

4 Zimmermann 0 0 First, extensive empirical studies were performed about the limits to the use of connector road cross sections. In accordance with RAA (Guidelines for the Design of Motorways, 00), connector road dimensions are a function of traffic volume and connector road length (see definitions of connector roads cross sections (Q to Q) as basic information in Figure ). This interdependence was a constituent also of previous codes and regulations and is based on studies conducted in the 0s and 0s. Two-lane cross sections are planned, on the one hand, as cross sections with hard shoulders (Q) when specific traffic volumes are exceeded and, on the other hand, nearly irrespective of traffic volume, as cross sections without hard shoulders (Q) for connector road lengths over 00 m. This research project is to examine systematically the limits to use, so far laid down in codes and regulations (RAA and the German Highway Capacity Manual (HBS ()), as a function of traffic volume and adapt them to present-day boundary conditions in road traffic. Moreover, the need for two-lane cross sections (Q) was investigated, but only because of the length of the connector road. In this connection, also the cross sectional width used and the impact of a hard shoulder on driving behavior in single-lane connector roads was analyzed independent of traffic volume. On the basis of observations made in the studies about dimensioning of cross sections, also the alignment of connector roads was assessed on the basis of speed and lane-keeping profiles. Section of the RAA (00), among other things, explains the basic principles of alignment of connector roads. As a rule, these provisions are quoted verbatim from the previous code, RAL- K- (), where only very few definitions about design elements and their consequences are laid down. This research project therefore investigated in more detail the boundary conditions with respect to traffic safety which are important in designing connector roads. METHOD OF INVESTIGATION This study is based on more than 0 connector roads in Bavaria, Baden-Württemberg, Hesse and North-Rhine Westphalia. In agreement with the client and the group in charge, respectively, appropriate sub-collectives were established for each individual problem. The research contractor (ISE) put particular emphasis on a relatively accurate collection of the alignment data (horizontal alignment, vertical alignment and cross section) in the section of connector road considered. For this purpose, the ISE used D laser scanning techniques to measure the lane area in the specific area under investigation. To detect the speed and lane-keeping profiles in the relevant areas of connector roads, a measuring system was used which records the approach of vehicles (objects) by measuring from a point of observation situated in the environment of the lane section under observation (monitoring delineator, Figure ).

5 Zimmermann Figure : Monitoring delineator with uncovered measuring device (left), example of picture with different driving lines and faded lane markings (right) 0 In another step, superposition upon the alignment data (horizontal alignment) determined the trajectory in the cross section relative to the right edge of the lane so as to obtain information about speed and lane-keeping profiles and, in this way, finally allow information to be gained about driving profiles in general. The evaluation of lane-keeping profiles was performed on the basis of diagrams representing, in the upper part, the evaluations of driving lanes, in the middle, the curvature band of a connector road section recorded by the D laser scanner and, in the bottom part, the evaluations of speed profiles of the vehicles evaluated (case in Figure ). Each abscissa shows the chainage of the center line of the road rising in the direction of movement. The lines shown in grey refer to the center point of the front axle of a vehicle. The 0 th percentile line is the median value of these individual driving profiles of all passenger cars. The th percentile line (on the right and left, respectively) describes the tracks of the outer wheels of the passenger cars evaluated, that is to say, % each of the vehicles drive with their left and right wheels, respectively, outside this line. Passenger cars were assumed to have a standard width of.0 m. The trucks evaluated on a smaller scale, and the passenger cars driving on the left lane of two-lane connector roads, respectively, are shown only in dashed lines indicating one-off trips. Because of a restricted size of the collective (data) and for better clarity, the percentiles of these vehicles are not shown. The connector road speed, V R, was determined according to the RAA (00) for the respective radius, and compared with the actual driven speed. V R corresponds to the speed which can be used for dimensioning on a wet course for the radius considered. However, the measurements were performed on a dry road surface. To ensure correct interpretation of the results, a procedure analogous to the calculation of V R was used, namely determining a reference speed, V tr, dependent on the respective radius with the same degree of utilization, n = 0.0, but with a

6 Zimmermann 0 0 Figure : Trajectories, band of curvature, and speed profiles at the Karlsruhe-Mitte junction, R. clearly higher, speed-independent adhesion coefficient, f R = 0., and additionally entered into the diagrams showing speed profiles. To deal with capacity issues, side-scan radar units were used in addition to the ELp, and the data gathered in this way were presented in q-v-diagrams. Both measurement systems were installed in delineators, which made them invisible to motorists, with the consequence that an unbiased mode of driving could be assumed. RESULTS OF THE INVESTIGATION The investigations of the limits to the use of connector road cross sections as a function of traffic volume resulted in these findings: None of the single-lane connector roads considered in this study registered any traffic breakdowns, although peak volumes of.00 passenger car units/h or.0 passenger cars/h were measured, which is clearly above the previous limit of.0 passenger cars/h considered for single-lane connector roads. The space mean speed V m, of the passenger cars remains more or less constant irrespective of traffic volume and type of vehicle. On connector roads, both in high and in low traffic volume conditions, motorists are used to keeping insufficient safety distances. The percentage of cars with insufficient spacing increases almost linearly with the traffic volume. However, no negative impacts on the accident rates of this tailgating profiles were found.

7 Zimmermann 0 0 The empirical investigations conducted within this research project show that no capacity bottlenecks must be expected on connector roads, provided that entries and exits as well as weaving areas are dimensioned properly. In future dimensioning, it is not considered necessary to have a separate look at connector roads. Irrespective of traffic volume, the following aspects were examined as well: Design of cross sections: - The cross section width used as well as the impacts of hard shoulders on driving profiles on single-lane connector roads of cross section Q; - the need for two-lane cross sections (Q) only because of the connector road length. Connector road geometry: - Evaluation of the geometry of connector roads on the basis of speed and lane keeping profiles. The width of the cross section of single-lane connector roads varies considerably in reality. They are only rarely corresponding to the value indicated for Q in the RAA (00). The lane width which is actually used differs from connector road to connector road. No particularly striking impacts of lane width on driving profiles and accident patterns were found. The lane width for cross section Q is determined in the RAA (00) to be.0 m. It is seen to be a good measure of a standard lane width irrespective of geometry elements of the respective connector road, and should be retained for that reason. Especially for operational reasons, a hard shoulder of.0 m width is recommended for all connector roads of group I, as both theoretical calculations and a practical test have shown the advantages of this solution. Moreover, a lane of.0 m width plus a hard shoulder of.0 m width can be re-marked where necessary (e.g. under conditions of increased traffic volume because of a detour due to construction work) without any additional lane broadening to achieve a two-lane cross section with a lane width of.0 m each (currently corresponding to the Q cross section). However, in this case, it should be taken into account that the associated entries and exits also have to be upgraded. It was also seen that drivers on loop connector roads are more likely to drive closer to the right edge of the lane, and often also used an existing hard shoulder. Consequently, on these connector roads, it is deemed to be more meaningful to apply a marking to the hard shoulder on the left side of the lane in order to improve the view of road operation service vehicles or vehicles suffering breakdowns.

8 Zimmermann 0 0 One alternative to the recommended change in dimensioning cross sections of Q consists in broadening the lane only in sections of narrow radii or move the crash barriers further away, always taking into account safety aspects. If connector roads without or with only short straight lengths require frequent changes in width because of the necessary broadening of curves, it may be more economical to ensure continuous broadening of the lane. Both investigations either derived from the analysis of accidents or the tendencies about the driving behaviour show a good agreement on the danger points. Accident analysis allowed a total of accidents to be evaluated. Accidents occurring on connector roads are mostly accidents due to driving errors (type- accidents). Both the evaluations of driving profiles and the findings derived from accident analyses show that two-lane Q cross sections in four-way interchanges have no advantages at low traffic volumes; in fact, they may even be a drawback with respect to traffic safety. It was found that the availability of a second lane is hardly used by drivers in connector roads with low traffic volumes. Measurements indicated that most passenger cars and also trucks operating on the right lane drive at a speed adequate to the geometry of the lane, thus causing no pressure to pass other vehicles. One of the original main reasons for a second lane thus no longer appears to apply. The left lane generally causes drivers to drive fast and, in combination with entry type E (left lane converging before the entry) seems to have disadvantages in terms of traffic safety. As a consequence, a two-lane cross section is felt to be meaningful in connector roads of motorway interchanges only in combination with a two-lane entry for reasons of capacity (in RAA (00), this is cross section Q). The impact of alignment on driving profiles and accidents was examined next, irrespective of connector road cross section. Analysis of driving profiles shows that the sequence of radii is clearly more important with respect to driving profiles as had been recognized so far or laid down in codes. It became evident that definitions had to be laid down for the relation of radii and the limitation of straights, but also that elements, which permit clearly higher speeds than those which can be negotiated at the crown radius, should generally be avoided. A clear influence of the type and geometry of connector roads on the accident is found. Among single-lane connector roads, it is the adapted loop connector roads which show the highest accidents rates. In case of two-lane connector roads, it is the direct non-adapted connector road which had the highest accident rates, although it must be seen that the population of loop connector roads with two-lane cross sections is too small to allow a reliable comparison with respect to the types of connector road. All in all, accident data show that the alignment is clearly more important than considered in the codes so far. In particular, outstanding phenomena in adapted loop connector roads and direct connector roads with adverse radius relations and/or long straights in between indicate that the

9 Zimmermann 0 0 lack of definitions of relations of radii and length of straights, which so far have been available in the RAA (00) only for open road sections, should be reconsidered. Accident analysis also indicated that the involvement of trucks on connector roads generally is clearly lower than on motorways in general, while the involvement of motorbikes is higher. While the number of accidents involving motorbikes on motorways in general is not particularly high, there are indications that the risk is considerably higher on connector roads under certain geometric road conditions, especially for accidents with severe passenger injuries. A rough check of the types of connector roads affected showed them to be generally loop or semi-direct connector roads; however, these interrelations would need more detailed studies to back their reliability, as the data considered here contains only a small sample. PRACTICAL IMPLICATIONS The investigations carried out in this research project were focused exclusively on type- connector roads (grade-separated / grade-separated). Applicability to group II connector roads (grade-separated / at grade) is possible in principle, but should be verified with further studies of connector roads within that group. As a consequence of the findings made in this research project with respect to the dimensioning of cross sections of connector roads, revising the dimensions of cross sections in the RAA should be recommended in accordance with Figure. The following principles for the design of connector roads of group I are recommended: - Direct connector road geometry preferably should be designed so as to promote constant speed. The optimum relation between consecutive radii is R :R :, while a ratio of R :R > should be avoided. - Radii R < 0 m should be avoided as they only allow connector road speeds of V R < 0 km/h which, in reality, are exceeded by far by most vehicles even in smaller radii. - Straights invite higher speeds. Where they are unavoidable, a certain length should not be exceeded or the following radius should be adapted. In this way, accelerating to speeds above the V R of the following radius is to be prevented. The new findings made in the research project regarding alignment aspects were summarized in two diagrams (Figure ) and are recommended for acceptance into the RAA. As accident analysis shows an increased involvement of motorbikes with a trend towards accidents with major passenger injuries, measures to be derived to protect motorbikes on the connector roads require an in-depth analysis of a sufficiently large population which, in particular, should include the dependence of motorbike accidents on geometric road design.

10 Zimmermann Q -I,0 hard shoulder,0 0, Areas of application In connector road group I: Combined with exit types A, A, entry types E, E, Weaving area types V, VR For loop connector roads, hard shoulder on the left Q -II In connector road group II: Separately designed exit and entry connector roads with l parallel < m In connector road group I: Two-lane weaving areas without hard shoulder In connector road group II: Q >,0 vehicles/h con.r. Only in connector road group I: Combined with exit types A, A, A, A, A, A Entry types E, E Also for two-lane weaving areas with hard shoulder Only in connector road group II: Combined designed exit and entry slip roads that run parallel to each other over the distance l parallel > m Figure : Recommended limits for the use of connector road cross sections in motorway junctions; basic reference: (RAA; 00) As the research project was focused on connector roads, effects of driving behavior on connector roads upon entries further down the road were not considered. However, it was observed that the horizontal alignment at the end of a connector road has a major influence on the speed of vehicles at the beginning of weaving into the main lane and, consequently, on the conflict potential created. However, this influence has not been reflected in the design of entries in accordance with RAA (00) so far, as this is independent of route geometry of connector roads. As the research contractor feels that this aspect has a considerable influence on traffic safety in entries, a recommendation is made about updating the RAA based on a study of the interrelations between driving profiles on connector roads within a short distance before an entry and driving behavior, and traffic safety, respectively, in consecutive entries.

11 Zimmermann 0 00 :. Recommended area for all sequences of radii : 00 0 : Curve radius R [m] 0 Curve radius R[m] 00 0 Recommended area To be avoided 0 to be avoided Curve radius R [m] Length of straight L G [m] Figure : Recommended limits for the use of radii and straights of connector roads in motorway junctions. REFERENCES. Roos, R., Zimmermann, M., Cindric-Middendorf, D. Fresh knowledge concerning the areas of use and the designing of ramps in accordance with the Guidelines for the Design of Motorways (orig. Neue Erkenntnisse zu den Einsatzbereichen und zum Entwurf von Rampen gemäß den RAA); Series Forschung Strassenbau und Strassenverkehrstechnik, Nr., 0. Road and Transportation Research Association (Forschungsgesellschaft für Straßen- und Verkehrswesen, FGSV): Guidelines for the Design of Motorways (orig. Richtlinien für die Anlage von Autobahnen, RAA), Cologne, 00. Road and Transportation Research Association (Forschungsgesellschaft für Straßen- und Verkehrswesen, FGSV): German Highway Capacity Manual (orig. Handbuch für die Bemessung von Strassenverkehrsanlagen, HBS), Cologne, 00