The effect of vehicle length on traffic on Canadian two-lane, two-way roads
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In April 1990, the Transportation Association of Canada initiated a project to evaluate the effects of overall truck length on safety and level of service on two lane, two-way roads in Canada. The specific objectives were: 1) to determine if current intersection design and signal timing practices are adequate for vehicles longer than those presently permitted, and 2) to determine if current passing sight distances and pavement marking practices are adequate for overtaking vehicles longer than those presently permitted. Specific emphasis was placed on the implications of extending overall vehicle maximum length from 23 to 25 metres. The key findings were as follows: 1) an increase in maximum vehicle length from 23 to 25 metres would have little significant impact on the operation of intersections; 2) the impact of truck length on passing operations on two-lane highways is dependent on the geometric design standard and operating speed of the highway, and as the operating speed increases, the sight distance required to pass vehicles also increases; 3) at operating speeds over 80 km/h, the existing passing zone pavement markings may not provide adequate sight distance for passing 23 metre trucks, and an increase in maximum vehicle length to 25 metres would marginally increase this inadequacy; and 4) standards for passing sight distance and passing zone markings have traditionally been established based on one car passing another car, and it is recommended that these standards be reevaluated to take into account the maximum length of truck combinations in the Canadian fleet.Keywords:
Overtaking
Sight
Geometric design
Daylight
Design speed
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Highway shoulders are still the most neglected aspect of road design, construction, and maintenance, and tend to be poorly maintained. This literature on the operational effects of shoulder width is not very extensive. This paper presents a study of the effects of shoulder condition on traffic flow on four sections of main road in India. The shoulders studied were classified into good, average, bad, and worst. Data on traffic speed, traffic flow, and vehicle positions were recorded on each section. The average speed of single vehicles, and the average reduction in speed of a vehicle during a crossing or overtaking manoeuvre, were measured in situ; crossing operations were observed for cars and heavy vehicles only. Effective carriageway width is reduced if vehicles cannot use shoulders during passing and overtaking; this is more important on single and intermediate lane roads, where vehicles are often forced to get on and off the paved area. Data are given about the observed reductions in effective carriageway width and in speeds of cars and heavy vehicles, due to poor shoulder condition. The results are presented as tables. The authors recommend the development of a speed-flow relationship for roads with various shoulder qualities, to find out about the combined effect of width and speed reduction on capacity.
Overtaking
Carriageway
Traffic speed
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The maximum permitted length of trucks on Canadian roads has increased significantly over the past 20 years. This has led to concern regarding the interaction of trucks with other elements in the traffic stream. This paper presents a summary of the effect of increased vehicle length on passing operations and, specifically, it looks at the effect on passing‐sight‐distance requirements of increasing the overall maximum vehicle lengths from 23 m to 25 m. Passing‐sight‐distance requirements were calculated based on generalized versions of models developed by Lieberman, in 1982, and Glennon, in 1988. The inherent random nature of vehicle and driver characteristics were incorporated into the passing model by conducting a discrete stochastic simulation of the passing maneuver. The results of the modeling exercise indicate that passing‐sight distances are considerably greater when passing a long truck than when passing a car. However, whether the truck is 23 m or 25 m long does not have a significant impact on the passing‐sight‐distance requirements.
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A past research effort investigated perceptual countermeasures using pavement marking patterns in New York, Mississippi, and Texas. The study showed that placing the markings at 4 bars per second were effective in reducing vehicle speeds. The purpose of the current study is to investigate differences between four different pavement marking spacing designs (constant, exponential, 2 bars per second, and 4 bars per second) in a driving simulator. In addition, the study evaluates vehicle lane placement to determine whether or not the markings can cause drivers to have fewer encroachments toward the center line which could potentially have a safety benefit on two lane roads. The study found no significant difference between the various treatments applied to the simulated roadway. However, an analysis of vehicle speeds at the end of the treatment area to see if they had reached the desired speeds showed that in all cases where a reduction was required without stopping, the observed speeds were within a 95% confidence interval of the desired speeds. An analysis of lane position was performed, and for the two bars per second and four bars per second design alternatives, the driver did travel significantly further away from the centerline. Additional research in the field may quantify the magnitude of the effect as well as possible safety implications in reducing crashes on two lane roads.
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Line (geometry)
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Variables in automobile design are discussed in relation to the present practices of marking no-passing zones on two- lane highways. One of the advantages in the recent increase in the horsepower of passing cars is the ability to complete passing maneuver in less time, thus reducing the possibility of being caught in the left lane of a 2-lane road with an oncoming vehicle rapidly reducing the time interval between life and death. This is closely allied with the lower height of the driver's eyes in the newer cars which, under certain highway conditions, reduces the distance that the driver can see a clear road ahead. Data were collected from a comprehensive series of investigations of passing practices on 2-lane highways. Detailed data were recorded for a total of 21,000 passing maneuvers at 32 locations in 7 states between 1938 and 1940. Data were obtained during recent studies by manual observations. The speeds of both the passed and passing vehicles were higher in 1957 than in 1938. However, the time spent in the left-hand lane by the newer vehicles in 1957 was 0.5 sec shorter than the time in 1938. The distance traveled in the left lane, increased 100 feet. It is concluded that there is little evidence to indicate that present practices of marking no-passing zones should be changed due to the changes that have taken place during the past years in vehicle design and driver performance.
Sight
Horsepower
Crest
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A highway need study was conducted in west virginia by reviewing speed studies on highways throughout the state. The operating speed on an existing highway is affected by design speed, traffic volume, and number of lanes. Also, for 2-lane roads, it is affected by the availability of sections on which the sight distance is of sufficient length to perform passing maneuvers safely. In the design of a new highway, it is the one factor which together with the traffic volume and assumed design speed determines the needed geometric features of a highway. An engineering analysis of the highway capacity requires the following information: (1) type of terrain through which the highway is located, (2) average highway speed and frequency of occurrence of sharp curves that cause abnormally low speeds, (3) percentage of the highway on which the passing sight distance exceeds 1500 feet, (4) the average truck factor and the truck factor on all long or steep grades, and (5) cross-section items such as shoulder and surface type, width, and condition. These items were determined for all highways in west virginia expected to carry annual volumes in excess of 1800 vehicles per day within the next 20 years.
Design speed
Geometric design
Sight
State highway
Highway Engineering
Operating speed
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The major purpose of this study was twofold: (a) investigate the physical components of the overtaking maneuver on two-lane rural highways and the relationship between these components, and (b) evaluate whether current practices in highway design are adequate. Data for the study were collected by two instrumented vehicles driven simultaneously along two rural-road sections: the first car, the impeder, was overtaken by the second, faster car. Findings regarding the initial maneuver distance and time, space and time gap at encroachment onto opposing lane, distance traveled and time in opposing lane, space and time gap on completion of maneuver, speed differences, and acceleration characteristics are evaluated and discussed. Comparison of the findings with AASHTO values is presented, and it is concluded that the values given by AASHTO may be somewhat conservative. It is concluded that the passing maneuver should be studied more extensively, with the intention of re-reviewing current road-design guidelines.
Overtaking
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The purpose of this study was to review road geometric standards (which are currently based on passenger car characteristics) taking into account truck driver eye-height and braking distance. A survey of truck driver eye-height gave a 5th percentile height of 1.87 m. Owing to the lack of empirical data on the braking performance of trucks, a theoretical approach was used to estimate design braking distances on the basis of the maximum legal braking distances of South African trucks. In addition, two American surveys provided further data on truck braking distances. A comparison between the speeds of cars and trucks in 1981 and 1982 on Transvaal freeways gave a consistent 17 km/h difference between similar percentiles. It was thus possible to compare the elements of road geometric design based on stopping sight distance for passenger cars travelling at a given speed and trucks travelling at a 17 km/h lower speed. It was found that, at design speeds above 100 km/h, the current standards for stopping sight distance, k-values for sag vertical curves and lateral clearance on horizontal curves were insufficient for trucks. Revised values for design speeds above 100 km/h, termed recommended values, are proposed for use until South African truck braking distances are known. (Author/TRRL)
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Braking distance
Geometric design
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A NUMBER OF CAPACITY STUDIES WERE CONDUCTED ON SOME OF THE MOST HEAVILY TRAVELED RURAL HIGHWAYS IN NEW YORK AND NEW NEGLAND DURING 1934 AND 1935. VEHICLES WERE CLASSIFIED AS A PASSENGER CAR, BUS OR TRUCK. EACH TRUCK WAS FURTHER CLASSIFIED AS TO WEIGHT AND TRAILER TYPE. FROM THE DATA THE SPEED OF EACH VEHICLE, ITS TIME OR DISTANCE SPACING FROM OTHER VEHICLES, AND THE EXACT VOLUME OF TRAFFIC IN EACH DIRECTION DURING ANY DESIRED TIME PERIOD COULD BE OBTAINED. DISTANCE SPACINGS WERE COMPUTED FROM TIME SPACINGS. THE THEORETICAL MAXIMUM NUMBER OF VEHICLES PER HOUR THAT ONE TRAFFIC LANE COULD ACCOMODATE AT VARIOUS SPEEDS WAS COMPUTED. IN MOST INSTANCES THE STUDY SHOWED A GRADUAL DECREASE IN AVERAGE SPEED WITH AN INCREASE IN VOLUME. IT WAS APPARENT THAT A MEASURE OTHER THAN AVERAGE VEHICLE SPEED ALONE MUST BE USED TO DETERMINE THE WORKING CAPACITIES OF HIGHWAYS. AS THE TRAFFIC VOLUME INCREASES THERE IS AN INCREASING TENDENCY FOR THE SPEED OF THE INDIVIDUAL VEHICLES TO BE GOVERNED BY THE SPEED OF THE PRECEDING VEHICLES, CAUSING A MARKED DECREASE IN THE MEAN DIFFERENCE OF SPEED BETWEEN SUCCESSIVE VEHICLES, ALTHOUGH THE DECREASE IN AVERAGE SPEED MAY BE SLIGHT. THE MAXIMUM NUMBER OF VEHICLES PER HOUR THAT CAN TRAVEL OVER VARIOUS RURAL HIGHWAY TYPES BEFORE ALL VEHICLES MUST START TRAVELING AT THE SAME SPEED AS THE PRECEDING VEHICLES WAS COMPUTED FOR SEVERAL LOCATIONS. OBSERVATIONS SHOWED THAT ALTHOUGH THE PERCENTAGE OF VEHICLES TRAVELING BELOW THE AVERAGE SPEED VARIES FOR DIFFERENT HIGHWAYS, THE PERCENTAGE TRAVELING BELOW THE AVERAGE SPEED ON ANY PARTICULAR HIGHWAY DOES NOT CHANGE APPRECIABLY WITH A CHANGE IN TRAFFIC VOLUME. VEHICLE TIME SPACINGS FOR A NUMBER OF LOCATIONS WERE STUDIED.
Trailer
Traffic speed
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Pavement markings have definite functions in a proper traffic control system. They are applied for the purposes of regulating and guiding the movement of traffic, and promoting safety without diverting the driver's attention from the roadway. It was observed that pavement markings located to the right of the car are detected more easily and at distances farther away when compared with the corresponding markings placed to the left of the car. However, compared to other types of longitudinal markings, the effect of edge lines on safety and driver behavior has been much less investigated. The conducted crash statistic analysis found that edge-line treatments on rural two-lane roadways may reduce accident frequency up to 26 percent and the highest safety impacts occur on curved segments of roadways with lane widths of 9 to 10 feet. The next stage was focused on complex investigations of edge lines impacts on driver behavior and reactions, including vehicle navigational and positioning issues, speed selection, and effect on driver visual perception. Stationary traffic observation, test driving, and several laboratory experiments were conducted on the selected rural two-lane highways with different roadway width before and after edge lines placement. Studies indicated that edge line treatments increase speed on average by 5 mph or 9 percent on both straight and curved highway segments; moves vehicles toward the pavement edge at both daylight and darkness in an average of 20 inches; reduce vehicle fluctuation around trajectory center line by 20 percent; reduce driver mental workload; improve driver's estimation of roadway curvature; and increase driver's advance time of intersection identification.
Daylight
Visibility
Line (geometry)
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This report describes the path of vehicles following a collision with another vehicle at a rural intersection. Detailed information from in-depth investigations of 70 intersection crashes was analysed. Rear end crashes at intersections were excluded as were collisions involving a motorcycle. The vehicle which had right of way most commonly had an impact speed of between 80 and 99 km/h and the impact point was on the front of the vehicle. The vehicle which was required to give way most commonly had an impact speed of between zero and 20 km/h and was struck between the front of the vehicle and the B-pillar. After the vehicle to vehicle impact half the vehicles travelled more than 18 metres, 20 per cent more than 34 metres and 10 per cent more than 50 metres from the centre of the intersection. The most common direction of the vehicle following the initial impact was found to be between 15 and 29.9 degrees, where the original direction of travel of the through vehicle is at zero degrees. Intersection geometry, speed zone, impact point and mass ratio influence the nature of the post impact trajectory of the vehicles involved. As the results show a high number of vehicles travel a large distance at a shallow angle following an intersection collision, extending crash barriers on the through road (the road with right of way) right up to the intersection may have some benefit. Clear zones surrounding the intersection are also advisable and have an added benefit of increasing sight distance. Hazards can be assessed for removal or relocation by applying the results of this study.
Motor vehicle crash
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