Cox, Cole & Arndt 1 TITLE: DEVELOPMENT OF A NEW TIME-GAP BASED INTERSECTION SIGHT DISTANCE MODEL FOR HEAVY VEHICLES
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1 Cox, Cole & Arndt TITLE: DEVELOPMENT OF A NEW TIME-GAP BASED INTERSECTION SIGHT DISTANCE MODEL FOR HEAVY VEHICLES A paper prepared for the th International Symposium on Highway Geometric Design Submission Date: 0 March 0 Word count: Number of words in paper: Number of figures in paper: (0 words) Number of tables in paper: (0 words) PAPER DETAILS General topic of paper: Sight Paper is for a Technical / Research Topic CORRESPONDING AUTHOR S DETAILS Mr David M. Cole Principal Designer (Civil) Queensland Department of Transport and Main Roads, Floor, Adelaide Street, Brisbane City GPO Box, Brisbane City, Queensland, Australia, 00. Phone: + 0 Fax: david.m.cole@tmr.qld.gov.au COAUTHORS DETAILS Mr Ricky L. Cox Director (Special Projects - Road Design) Queensland Department of Transport and Main Roads Phone: ricky.l.cox@tmr.qld.gov.au Dr Owen K Arndt Director (Road Design) Queensland Department of Transport and Main Roads Phone: owen.k.arndt@tmr.qld.gov.au Address and fax details are the same for all authors.
2 Cox, Cole & Arndt 0 ABSTRACT In Australia, three sight distance models are currently used to provide sufficient sight distance on each of the legs at unsignalized intersections. The intents of these models are to ensure drivers have suitable sight distance and visibility angles to: Assess the intersection layout. Assess critical acceptance gaps and be able to undertake the relevant maneuvers. Historically, these sight distance models have covered only car operation. Truck operation has been assumed to be implicitly covered due to greater truck driver eye height and lower operating speed. Since 00, two of the models have been adapted for truck stopping capability. However, this is essentially a stop gap measure until a new intersection sight distance model is developed for trucks based on observed driver behavior and safety performance. In 00, AASHTO adopted time-gap based intersection sight distance models as a result of NCHRP Report. However, given the different heavy vehicle types used in Australia and the requirement to explicitly design for both car and truck operation, it was realized that gap acceptance times for Australian heavy vehicles would need to be established. This paper describes the development so far of a new time-gap based intersection sight distance model to suit the full range of Australian vehicle types including m and m long road trains that are unique to Australia. The paper also describes the field research undertaken and the need for supplementary computer modelling because of the limited sites with sufficient numbers of heavy vehicles to make gap analysis practical.
3 Cox, Cole & Arndt 0 0 DEVELOPMENT OF A NEW TIME-GAP BASED INTERSECTION SIGHT DISTANCE MODEL FOR HEAVY VEHICLES INTRODUCTION Sight distance at an intersection is fundamental to both safe and efficient operation of the intersection. Drivers must clearly be able to see: The path they have to follow; the position of holding lines, lane lines and turning lines; points of conflict with other users; prospects of a collision occurring; and gaps in the traffic of sufficient length to be able to make the proposed maneuver safely and without undue impedance to the movement of other vehicles. Consequently, in Australia, three sight distance models are currently used to provide sufficient sight distance and visibility angles on each of the legs at unsignalized intersections and desirably at signalized intersections. These models are described in the national Austroads design guide for intersections () and discussed in a later section. Initially, these sight distance models explicitly covered only car operation. Truck operation was assumed to be implicitly covered due to greater truck driver eye height, lower operating speed or, in some cases, compatibility with truck sight distance requirements in earlier design guides ((), ()). With the 00 design guide (), two of the models were adapted to cover trucks. However, this was essentially a stop gap measure because the 00 design guide () introduced the need to explicitly cover designing for the appropriate trucks as well as cars for all roads. It was realized at the time that the adaptions had limitations and that a new intersection sight distance model had to be developed for trucks based on observed driver behavior and safety performance. This paper covers the limitations of the current models and the development to date of new time-gap based intersection sight distance models for both cars and trucks. Although it was known that AASHTO () adopted time-gap based intersection sight distance models as a result of NCHRP Report (), it was realized that simply adopting AASHTO models was not appropriate because of the very different range (type and size) of heavy vehicles used in Australia (see Figure ).
4 Cox, Cole & Arndt.m RIGID TRUCK (.m).0m TYPE ROAD TRAIN PRIME MOVER & SEMI-TRAILER (m) (General access vehicle).0m B-DOUBLE (m or m).m A-DOUBLE (TYPE ROAD TRAIN) (.m).m A-TRIPLE (TYPE ROAD TRAIN) (.m).m US Turnpike-Double Combination (.m) (WB-D [WD-D]) FIGURE Comparison of Australian heavy vehicles and largest US heavy vehicle. Development of time-gap based intersection sight distance models has not simply involved the collection and analysis of data similar to that described in NCHRP Report (). It quickly became apparent that sites with large traffic volumes on the major road and sufficient numbers of heavy vehicles on the minor road to make data analysis practical were limited in number and would be restricted to intersections with 0-0km/h speed limits. There were no practical sites for the large road trains that operate in the remoter areas of
5 Cox, Cole & Arndt the country. Therefore the paper also covers the development of a prototype tool that uses first principles to analyze the speed reductions of a vehicle on the major road given a time-gap and speed profile of a vehicle turning left or right into the major road. The tool will be used to supplement research gaps, test single gap entering maneuvers and will aid in development of the new intersection sight distance model. BACKGROUND The three sight distance models currently used in Australia are: Approach Sight (ASD) introduced in () to allow drivers to see sufficient details of the layout (road surface, line-marking, islands etc.) when approaching an intersection in order to decide what to do. It is a desirable requirement in some cases and mandatory in others. It is predicated on Stopping Sight (SSD) to the road surface for cars and trucks. Safe Intersection Sight (SISD) introduced in () to allow drivers sufficient visibility in order to undertake any of the maneuvers associated with an intersection of a major road and a stop or giveway controlled minor road. Being the primary model, it is also intended to allow for greater driver workload at intersections and unexpected stopping for stalled vehicles. Consequently the resulting sight distances are derived from a combination of observation time, perception-reaction time and emergency stopping, but the distances are really intended to allow drivers sufficient time to make a maneuver with an implicit level of safety and for major road vehicles to normally proceed with little or no impedance to travel speed (except obviously for a vehicle turning from the major road into the side road). Originally the SISD model was expressed in terms of a table. It is only in the 00 guide () that the SISD model was accompanied by equation (). DT V V SISD = ( d a) where Decision time D T = second observation time O T + perception-reaction time R T ; V = speed of vehicle (km/h); d = coefficient of deceleration; and a = grade (%) ( + upgrade, - downgrade ). Minimum Gap Sight (MGSD) introduced in 00 () to allow all car drivers to see, and be able to readily judge, critical gaps in the traffic stream (or see sufficiently far) in order to abandon or proceed with a maneuver. Besides the sight distance requirements for intersections (), it is also necessary to meet the normal stopping sight distance requirements for a small hazard on the road for both cars and trucks (). This is readily met if the ASD model is met. Application of all these models includes driver eye height and position and object height and position as specified in the respective design guides ((), ()). The MGSD model also sets visibility angle limits. The ASD and SISD models evolved from specifications and models in earlier design guides ((), (), ()), some of which in turn were derived from data in the American AASHO Policy () as shown in Table. The evolution of Australian intersection sight distance models, has been compiled in detail by Cox (). ()
6 Cox, Cole & Arndt TABLE National Association of Australian Road Authorities (NAASRA) Crossing Sight (), Austroads Entering Sight () and Safe Intersection Sight () with time-gaps added Design Speed (km/h) Pre Crossing Sight Model () Post Entering More Important Road has More Important Road has Sight two running lanes four running lanes Model () Sight (.m to.m) from driver of stopped car (m) Timegap Sight (.m to.m) from driver of stopped truck (m) Timegap Sight (.m to.m) from driver of stopped car (m) Timegap Sight (.m to.m) from driver of stopped truck (m) Timegap (m) Timegap Post Safe Intersection Sight Model (), () Rural (m) Timegap Urban (m) * * * * Limiting value based on assumption that drivers unlikely to seek gaps greater than 00m. Note that distances assume: Level pavements Minor road vehicle stopped m (m min.) back from conflict point Measurement along center of path of major road vehicle. Time-gap = time to travel distance at design speed. Pre Crossing Sight s derived from Fig. VIII- and VIII- in AASHO () and based on no impedance to speed of vehicles on more important road. Pre Crossing Sight s considered to be suitable for vehicles to turn left or right into major road from side road. NAASRA is the predecessor of Austroads. When the SISD model was first introduced in (), it was accompanied by the Entering Sight (ESD) model. This model was intended to ensure that sufficient sight distance was available for a car to turn left or right into the major road from a stopped condition in the minor road without impeding a vehicle travelling at the design speed on the major road. This model was adapted with minor changes from AASHTO (). However, this model was rarely used because its application was discretionary and the resulting sight distance was often impractical to achieve. This model was dropped in the 00 design guide (). The model is included in this paper because it has a bearing on the appropriateness of a single time-gap model for entering maneuvers. Without the benefit of any technical paper on the development of the SISD model, it is necessary to examine the resulting sight distances against other traffic performance measures of the time. The Austroads 00 () Critical acceptance gap for cars column in Table shows gap acceptance times for cars as used for the analysis of intersection performance since the early 0 s. These gaps were based on observed behavior and came from research undertaken by the then Australian Road Research Board (ARRB) on urban roads in the early 0 s (). These times form the basis of the MGSD model above. Timegap
7 Cox, Cole & Arndt TABLE Comparison of Austroads (), AASHTO () and TS Project ((), ()) time-gaps Crossing Movement Description Two-lane/one way Three lane/one way Four-lane/one way Two-lane/two way Four-lane/two way Six lane/two way Austroads (00) Critical Equivalent acceptance car gap by gap for SISD Model cars to. (depends on speed) AASHTO (0) Sight distance time-gap Cars..... TS (0) Critical acceptance time-gap Artic. m artic. trucks B-double Right-hand turn from major road Across one lane Across two-lanes Across three lanes to. (depends on speed) Right-hand turn from minor road One way Two-lane/two way Four-lane/two way Six lane/two way to. (depends on speed) Not interfering with A -0 NA Left-hand turn Requiring A to slow to. (depends on speed) Not interfering with A -0 NA Note: Diagrams reflect Australian practice of driving on the left. = no data available; NA = not applicable. A comparison of the equivalent SISD time-gaps in Table with the Austroads () and AASHTO () time-gaps in Table shows that the SISD model supports: Cars crossing of a two-lane road for all design speeds. Cars crossing of a four-lane road for speeds > 0km/h (this may include a median in the order of m width). Cars turning left from the minor road with some impedance to the speed of a vehicle on the major road. Note that the range of time-gaps for non-interference seems to have come from AASHTO (). Intuitively, larger time-gaps would be required for larger operating speeds since the acceleration rate of the entering vehicle decreases as its speed increases something which the SISD model provides.
8 Cox, Cole & Arndt 0 0 Cars turning right (across one lane for all major road speeds and across two-lanes for speeds > 0km/h) from the minor road with some impedance to the speed of a vehicle on the major road. Note that the range of time-gaps for non-interference seems to have been added from AASHTO () as with the left-turn entering maneuver. Cars turning right from the major road across two major road lanes for all major road speeds and across three lanes for speeds 0km/h. Cars turning right from the major road across a narrow median and two major road lanes for all major road speeds 0km/h. A comparison of the SISD time-gaps with the earlier Crossing Sight model gaps for trucks in Table shows that the SISD model was not intended to support heavy vehicle operation without any impedance to the speed of vehicles on the major road. The expansion of the ASD and the SISD models to cover truck operation as included in Austroads () was simply achieved by substituting truck stopping distances where the models used car stopping distance and a reduction in the decision time component of the SISD model. Consequently the truck version of the SISD model in Austroads () essentially yields the same time-gaps (and sight distance) for a given major road operating speed. Experience has shown that the SISD model yields sufficient sight distance for cars and some trucks. It is only for large vehicles that some doubts have been expressed by road designers. It is fair to say that the SISD model has been widely accepted and rarely questioned. Perhaps this is due to the somewhat emotive name and being seen to be closely related to the long used and readily accepted stopping sight distance model. Furthermore, it must be realized that the SISD model is not applied in isolation. The stopping sight distance model, MGSD model, approach visibility requirements and signage requirements all combine to help drivers make use of the available visibility. Even so, there are issues on which the SISD model can be challenged, namely: No explicit provision for the number of lanes on the major road indeed, it is deficient in some cases as identified previously. No adjustment for the minor road being at a skew angle to the major road. Grade adjustment issues - being expressed in terms of a stopping event for the major road vehicle, the SISD model yields the following adjustments for grade in Table :
9 Cox, Cole & Arndt TABLE SISD grade adjustment issues () Crossing Movement Grade adjustment by model Grade adjustment explanation Right-hand turn from major road None for side road, model only makes adjustment for major road grade Turning vehicle starts on uphill grade - positive adjustment Start on uphill grade gap reduced by model if uphill grade on major road but needs to be increased gap increased unnecessarily by model if downhill grade on major road Start on downhill grade gap increased unnecessarily by model if downhill grade on major road gap reduced by model if uphill grade on major road and likely to negate effect of minor road grade Gap increased for approaching major road vehicle on downhill grade Appropriate adjustment? Turning vehicle starts on downhill grade - adverse adjustment Gap reduced for approaching major road vehicle on uphill grade Right-hand turn from minor road Left-hand turn Turning vehicle turns onto uphill grade - adverse adjustment Turning vehicle turns onto downhill grade - unnecessary positive adjustment Turning vehicle turns onto uphill grade - adverse adjustment Turning vehicle turns onto downhill grade - unnecessary positive adjustment Gap reduced for approaching major road vehicle from left on uphill grade (but need to allow more time for entering vehicle to accelerate on uphill grade) Gap does not need to be increased (downhill grade aids acceleration) Gap reduced for approaching vehicle from right on uphill grade (but need to allow more time for entering vehicle to accelerate on upgrade) Gap does not need to be increased (downgrade aids acceleration) Note: Diagrams reflect Australian practice of driving on the left. TOWARDS A NEW INTERSECTION SIGHT DISTANCE MODEL Having established the shortcomings of the current Australian design models for intersection sight distance (especially with respect to heavy vehicle operation), the Austroads Road Design Panel proposed, as a starting point, a research project to establish gap acceptance times for heavy vehicles at intersections. This proposal was accepted and became part of Austroads research project TS covering Road Design for Heavy Vehicles. The use of gap acceptance times as a foundation for a possible new intersection sight distance model was predicated on a number of factors:
10 Cox, Cole & Arndt AASHTO () adopted time-gap based intersection sight distance models as a result of NCHRP Report (). However, given the different heavy vehicle types used in Australia, it was realized that gap acceptance times for Australia heavy vehicles would need to be established. A new primary sight distance model based on gap acceptance may overcome the need for the current Minimum Gap Sight Model or alternatively, may make the intent of this model more obvious. The time-gaps would reflect actual driver behavior which in turn would support any case for substantial changes in sight distance from current practice. In particular, the gap acceptance times automatically cover driver expectancy and its effect on the perception, reaction, decision and gear selection tasks. The interrelationship between these and the allowance that needed to be made for them was always a point of contention with earlier theoretical sight distance models ((), ()). Given the existence of the Minimum Gap Sight Model, which has correspondingly smaller time-gaps than in AASHTO ((), (), ()) for the basic two-lane, two-way major road cases (being.s to.s smaller as shown in Table ), it was originally assumed that project TS could concentrate on heavy vehicle gap acceptance times and simply confirm whether the AASHTO () car time-gaps would be more appropriate for a new design model. Furthermore, given the difficulty in obtaining suitable sites with sufficient heavy vehicle volumes in order to obtain statistically significant data, but at the same time satisfy viable data collection and analysis requirements, it was anticipated that supplementary data would have to be obtained from computer modelling or NCHRP Report (). Such supplementary data would include: Effect of grades and intersection skew angle on time-gaps Effect of extra major road lanes and median width on entering and crossing times since suitable multilane sites for collecting data were limited. Time-gaps for road trains since no sites with viable volumes for data analysis would exist it was anticipated that road train time-gaps could be developed with some guidance from the time-gaps for B-doubles. A further advantage of developing a supplementary computer model that allowed driver behavior to be analyzed with respect to left and right-turn entering maneuvers by trucks, was seen to be the ability to confirm whether a single time-gap was suitable for all major road speeds (as in AASHTO ()) or whether this gap should be speed dependent to some extent. NCHRP Report In NCHRP Report, Harwood et al () reviewed the then current AASHTO () intersection sight distance models (and their evolution), the extent of their adoption by the various US State road authorities and the practice in other countries. Particular attention was given to the left and right-turn entering maneuvers by vehicles stopped in the minor road at a major road / minor road intersection because the then current model was not universally accepted and was widely questioned (as was the case with essentially the same model in Australia). Before settling on a constant time-gap for the entering maneuvers, Harwood et al () examined the time-gaps yielded by the then existing AASHTO () model and the deceleration characteristics and acceleration characteristics of vehicles associated with entering maneuvers. The distances in the AASHTO model of the time are generally in the order of 0% of the values shown for ESD in Table and show a time-gap increasing with speed. In settling on a uniform time-gap based sight distance model for the entering maneuvers, Harwood et al () cite the finding of earlier studies by Kyte et al and Lerner et al that found that the critical time-gap does not vary as a function of the approach speed, which supports the practice of using a constant critical gap across all design speeds for a given vehicle. Furthermore, the supporting acceleration/deceleration study by Harwood et al () (for entering and major road vehicles respectively) covered a range of major road vehicle speeds. The mean speed was 0km/h and the th percentile speed was km/h. Australian Gap Acceptance Study for Heavy Vehicles As a prelude to developing a new intersection sight distance model, gap acceptance studies were undertaken at six sites in Australia as part of research project TS. Four sites were T-intersections and two sites were cross intersections. The sites were located in the vicinity of a major sea port or in an industrial area. This was to ensure that there were a high number of large trucks (especially on the minor road) with sufficient major road volume to give a suitable distribution of timegaps. All sites had conventional intersection geometry and had generous sight distance in order to avoid any effect that low sight distance may have on forcing drivers to accept smaller gaps than they would prefer. Conversely, it was expected that truck drivers would normally not reject a suitable gap when they could see that
11 Cox, Cole & Arndt they could wait for an even larger following gap (whereas this was observed for some car drivers). Data collection and analysis closely followed the procedures and experience detailed in NCHRP Report () and are reported in Bucko et al ((), ()). The data was for five distinct heavy vehicle classes. The five heavy vehicle classes analyzed in this study were medium rigid truck, heavy rigid truck, prime mover and semi-trailer, truck-trailer, and B-double, refer Figure. With the exception of the truck-trailer vehicle, any of these vehicles could be the relevant design truck for intersection sight distance, depending upon road access controls. The truck-trailer gap sizes were expected to complement those for the prime mover and semi-trailer. Data analysis using the Raff method ((), (), ()) yielded critical acceptance gaps, see TS columns in Table. However, there was insufficient data to be able obtain practical or accurate results for the Logistical Transformation (Logit) method () which determines the probability that a particular size gap will be accepted by a certain percentage of drivers ((), ()). For the larger vehicles, the critical acceptance gap was also able to be moderated in most cases (and needed to be, given the range for the critical gap) against the time-gap which no drivers rejected in order to set a time-gap that was suitable for intersection sight distance design refer Table. Computer modelling as described below was also undertaken to help clarify the observed results where necessary. An unexpected finding for most heavy vehicles making a left-turn entering maneuver (equivalent to AASHTO Case B () right-turn) is that at least the same if not more time is required than for the right-turn entering maneuver (equivalent to AASHTO Case B () left-turn) even though the later involves crossing all lanes to the right (see Table and Table ). The subsequent computer modelling showed that this can be attributed to the fact that the entering heavy vehicle has to turn more tightly and more slowly when first entering the major road. Computer Modelling Research Even though the NCHRP Report () established the validity of a constant time-gap (that is independent of major road speed) being suitable for left and right-turn entering maneuvers (AASHTO Cases B, B ()), it was realized that something more than citing AASHTO practice would likely be required to convince Australian practitioners that a single time-gap would also be appropriate for large vehicles especially since they had been used to a model that gave increased time-gaps for increased speeds. As discussed earlier, there was less than the preferred amount of data for some cases when determining the time-gaps for intersection maneuvers for the three longest vehicle types. And from this experience, it was found there would be no practical sites for road trains since these vehicles are largely restricted to low volume roads. As a result, a prototype tool was developed to supplement the unavailable data. For this paper, only the data for an average load (t) B-double (m) has been included for the right turn maneuver in order to demonstrate how the model will be used and compared with observed results and how it can be used to generate extra data. Specifically this covers the following cases: Right-hand turn through 0 degrees from the minor road onto a two-lane two-way major road (equivalent to AASHTO Case B left-turn ()) for speeds 0, 0 and km/h, refer Table ; and Crossing of two, four, and six lane two-way roads, refer Table. Methodology used to evaluate time-gaps STEP : Use VPATH to calculate and plot swept path details for the turning vehicle (). The VPATH program is used for the design or checking of turning requirements for vehicles in operation on specific sections of road e.g. turning roadways at intersections, roundabouts, property entrances etc.
12 Cox, Cole & Arndt Vehicle A 0 FIGURE VPATH output for a prime mover and semi-trailer superimposed on the intersection layout. Vehicle type INPUTS Minimum turn radius (0) Steering path geometry derived from vehicle plotted every one meter from stop line to location on the OUTPUTS intersection where vehicle starts to move away from reference line, refer Figure. STEP : Use VEHSIM program to simulate the performance of the turning vehicle over time (). The software accepts data that defines the type of vehicle and the horizontal and vertical geometry of the alignment along which the vehicle will be running. Vehicle type VEHSIM alignment defined with horizontal and vertical geometry INPUTS Start location of vehicle on VEHSIM alignment Vehicle start speed (0 km/h) Maximum speed around curve (km/h) (0) Time interval between calculated results (0. default) VEHSIM results file containing vehicle performance for the turn maneuver over time. OUTPUTS
13 Cox, Cole & Arndt STEP : Use Acceptance Gap Tool spreadsheet shown in Figure to evaluate the interaction between the entering vehicle (Vehicle B) and the approaching vehicle (Vehicle A) on the major road, as shown in Figure. The tool can be used to evaluate various scenarios using different inputs such as, left or right-turns, skewed intersections, steering paths, grades, initial speeds of Vehicle A, grade corrected deceleration rates, initial timegap and final closing headway and VEHSIM speed-time-distance profile data for Vehicle B. Vehicle A final speed between vehicles Gap time between vehicles Vehicle B speed FIGURE Spreadsheet acceptance gap evaluation tool. The process in the tool then uses a heuristic method to optimize the deceleration for Vehicle A up to a given maximum value to determine the optimum closing speed for the given initial gap time, closing headway and initial speed. This is achieved by: Vehicle A is moved in terms of time intervals (normally 0. seconds) set by the VEHSIM data for Vehicle B;
14 Cox, Cole & Arndt Vehicle A speed is initially unchanged for given perception-reaction time; Vehicle A is slowed at the maximum grade corrected deceleration rate, if necessary, until a speed is reached where Vehicle A continues at that speed and the vehicles do not come closer than the given closing headway. This is achieved by the macro searching ahead through the remaining VEHSIM data for Vehicle B for the speed and the recalculated closing headway. This is intended to model what a driver of Vehicle A would typically do in assessing the future movement of Vehicle B and making any appropriate adjustment; If Vehicle A cannot achieve any of the above criteria (including stopping), the time-gap is unsafe for the maneuver; and The calculated results of the gap criteria is displayed in the top right corner of the tool (refer Figure ). Derived distance of vehicle from stop line to location on the intersection where Vehicle B starts to move away from reference line Initial speed of Vehicle A (km/h) Major road grade (%) INPUTS Reaction/decision time of Vehicle A driver Applied braking deceleration applied by Vehicle A driver (m/s²) Initial time-gap between vehicles (the time-gap being evaluated) Final closing headway VEHSIM results file Notifies user if time-gap is or is not large enough for the given applied braking deceleration Final closing speed of Vehicle A (km/h) Final closing distance between vehicles (m) OUTPUTS Difference in initial and final speeds (km/h) Speed reduction (%) Computer Modelling Results Details of the computer simulation results for the entering and crossing maneuvers are given in Table and Table respectively. The main findings from the computer simulation follow. Right-turn from minor road onto a two-lane two-way major road It has been observed that: If Vehicle A decelerates as soon as possible more time is available for it to catch up to the entering vehicle. This minimizes any reduction in speed needed. Also in all high speed environments (0-km/h) scenarios evaluated, Vehicle A required some amount of speed reduction; o Even with a very favorable gap of 0s (speed > 0 km/h and level grade), Vehicle A has to slow a 0s gap being greater than what can often be practically provided; o Even when no slowing by the approaching vehicle was observed with the TS gap acceptance analysis, slowing downstream of the intersection had to occur for an entering heavy vehicle. Minor road grade has little effect on the speed of the vehicle mainly due to turn speed being limited by turn radius; Higher initial deceleration allows a higher final converging speed; If the major road grade is % - increasing the time-gap to compensate does not afford a significantly higher final speed for the converging vehicle; o Grade limits truck final speed to some constant value; o Increased time-gap simply gives more time to decelerate yet grade allows greater effective deceleration, in contrast to entering cars which can accelerate on grade (albeit more slowly); and Major road vehicle speed does have some influence on the time-gap but it is possible and practical to have a constant time-gap for entering heavy vehicles.
15 Cox, Cole & Arndt TABLE Computer modelling results for right-turn from minor road onto a two-lane two-way major road for a B-double (angle of turn 0 ) Vehicle A deceleration Vehicle A deceleration (.m/s ) (.m/s ) Major road Major road Minor road Time-gap final speed Time-gap final speed speed (km/h) grade (%) grade (%) (km/h) (km/h) Time-gap Time-gap 0 0 s s.s 0s s s.s 0s = time-gap is unsafe for the maneuver. Note: Turn angle is for a right-turn maneuver from a minor road onto a two-lane two-way major road. Decision/perception-reaction time is an input into the model ( seconds). Crossing Maneuver From Table it can be observed that: A B-double takes about 0. seconds for the rear of the vehicle to clear each additional lane. These results are in line with what AASHTO has stipulated for cars (). Type & Type Road Trains, take about 0. seconds for the rear of the vehicle to clear each additional lane. This apparent contradiction is due to the length of the vehicle, and the higher speed reached when the rear of the vehicle clears a lane. For practical purposes 0. seconds would be used for consistency. When the approach to the minor road is on an upgrade the relevant crossing time should be increased by 0. seconds per % grade for the B-double and Type Road Train and 0. seconds per % grade for a Type Road Train.
16 Cox, Cole & Arndt TABLE Computer modelling results for two-way major road crossing maneuver Crossing major road with.m lanes m % % % % %(R) %(L) L L Road Vertical curve (m) Shoulder (m) %(L) %(R) Number of lanes B-double (m) Crossing time Type Road Train (.m) Crossing time Type Road Train (.m) Crossing time No decision time added expect seconds based on TS gaps
17 Cox, Cole & Arndt TABLE Proposed sight distance time-gaps for Australian design vehicles Crossing Movement Description Two-lane/one way Three lane/one way Four-lane/one way Two-lane/two way Four-lane/two way Six lane/two way Car (=AASHTO)..... Sight Time-gaps Heavy Rigid Truck..... Semi- Trailer..... B- double.. Road Train (Type ). Road Train (Type )..... Right-hand turn from major road Across one lane Across two-lanes Across three lanes Right-hand turn from minor road One way Two-lane/two way Four-lane/two way Six lane/two way Left-hand turn All cases..... Note: Diagrams reflect Australian practice of driving on the left. Car time-gaps same as AASHTO values except for left turn where modelling results indicated a slightly higher value is required for higher speeds. Derived TS results shown in red Derived modelling results shown in green denotes unlikely to be encountered in practice
18 Cox, Cole & Arndt 0 0 CONCLUSION The current Austroads SISD model s added truck capability simply ensures suitable stopping capability for a truck on the major road rather than ensuring sufficient time for a truck to undertake the relevant intersection maneuvers. Despite its limitations, whoever developed the model knew what they were doing in terms of sight distance for cars. Unfortunately there is no technical reference material for the model, and given the lack of material, the model became misunderstood over time. A new time-gap based intersection sight distance model will have a more rigorous technical foundation especially for trucks. Having sight distance models with a rigorous and well documented technical foundation will give practitioners more scope to optimize intersection designs in situations where they are forced to make trade-offs between the various design parameters. Since it can be demonstrated that the time-gaps reflect actual driver behavior, it is anticipated that there will less resistance to changing to a new sight distance model even though there will be substantial changes in sight distance from current practice especially for trucks. Proposed design values for new intersection sight distance models are given in Table. It is likely that a version for existing intersections may also have to be developed, based on time-gaps where heavy vehicles cause some or more impedance to major road vehicle speeds. Due to the field research limited to 0-0km/h environments and insufficient data being captured for some of the larger vehicles (refer Figure ), computer simulation was used to supplement measured time-gaps (see Table ). The Acceptance Gap Calculation Tool that was consequently developed has also: Provided a greater understanding of vehicle interaction for the observed time-gaps; Verified the suitability of using a constant (that is, speed independent) time-gap for vehicles turning left or right onto the major road from the minor road; Shown where the current SISD and MGSD models have limitations. Furthermore there may be situations where a number of coincident factors (grade, intersection angle, number of lanes, turn radius, radius on the major road) may lead to over allowance by simply adding these factors together. For such situations, the Acceptance Gap Calculation Tool may become an essential design tool. The new model will achieve practical, yet justifiable results by: using gap acceptance times from Australian research (complemented by earlier American research) and supplemented by the prototype Acceptance Gap Calculation Tool; and using time-gaps that reflect actual driver behavior. The benefits of the new model are: replacement of two existing sight distance models (SISD and MGSD); and having a new model that will be simpler to use.
19 Cox, Cole & Arndt 0 0 REFERENCES. Guide to Road Design Part A: Unsignalised and Signalised Intersections. Austroads, Sydney, NSW, Australia, 00. Rural Road Design, A Guide to the Geometric Design of Rural Roads. National Association of Australian Road Authorities (NAASRA), Sydney, Australia,.. Interim Guide for the Design of Intersections at Grade. NAASRA, Sydney, Australia,.. A Policy on Geometric Design of Highways and Streets. AASHTO, Washington D.C., USA, 00.. Harwood, D.W., Mason, J.M., Brydia, R.E., Pietrucha, M.T., and Gittings, G.L., Intersection Sight, NCHRP Report. Transportation Research Board - National Safety Council, Washington D.C., USA,.. Guide to Traffic Engineering Practice - Part : Intersections at-grade. Austroads, Sydney, NSW, Australia,.. Guide to Traffic Engineering Practice - Part : Intersections at-grade. Austroads, Sydney, NSW, Australia, 00.. Guide to Road Design Part : Geometric Design. Austroads, Sydney, NSW, Australia, 00.. A Policy on Geometric Design of Rural Highways, AASHO, Washington D.C., USA,.. Cox, R.L., Evolution of Australian Intersection Sight Models. Queensland Department of Transport and Main Roads, Brisbane, Australia, 0.. A Policy on Geometric Design of Highways and Streets, AASHTO, Washington D.C., USA,.. Urban Road Design Manual (URDM) Volume. Queensland Department of Main Roads, Brisbane, Australia,.. Cox, R.L., Cole, D.M., Limitations of the Safe Intersection Sight Model: Why a new gap acceptance based model is being developed for Austroads. Presented at the South East Queensland Technical Forum, Nerang, Australia, 0.. A Policy on Geometric Design of Highways and Streets. AASHTO, Washington D.C., USA, 00.. A Policy on Geometric Design of Highways and Streets. AASHTO, Washington D.C., USA, 0.. A Policy on Geometric Design of Highways and Streets. AASHTO, Washington D.C., USA,.. Bucko, A., Ritzinger, A., Tziotis, M., and Eady, P., Austroads Report. Road Design for Heavy Vehicles, Progress Report : Gap Selection Criteria for Heavy Vehicles. Austroads Project TS. ARRB Research, Australia, 0.. Bucko, A., Ritzinger, A., Tziotis, M., and Eady, P., Austroads Report. Road Design for Heavy Vehicles, Progress Report A: Gap Selection Criteria for Heavy Vehicles. Austroads Project TS. ARRB Research, Australia, 0.. Graphics Manual, Chapter Vehicle Path (VPath), April. Queensland Department of Transport, Brisbane, Australia,. 0. Austroads Design Vehicles and Turning Path Templates Guide. Austroads, Sydney, Australia, 0.. Vehicle Simulation (VEHSIM). Queensland Department of Main Roads, Brisbane, Australia, 00.
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