Highway and Railroad Engineering: Lesson 7 PDF

HIGHWAY AND RAILROAD ENGINEERING: LESSON 7  COURSE OUTLINE Reference: CHED Memorandum Order No. 92 Series 2017  TRAFFIC FLOW FUNDAMENTALS INTRODUCTION TO TRAFFIC ANALYSIS Traffic Analysis o Traffic analysis provides basis in measuring the operating performance of the hi...

HIGHWAY AND RAILROAD ENGINEERING: LESSON 7  COURSE OUTLINE Reference: CHED Memorandum Order No. 92 Series 2017  TRAFFIC FLOW FUNDAMENTALS INTRODUCTION TO TRAFFIC ANALYSIS Traffic Analysis o Traffic analysis provides basis in measuring the operating performance of the highway o Primary function of a highway system is providing high level of transportation service (LOS) and safety INTRODUCTION TO TRAFFIC ANALYSIS Various dimensions used in traffic analysis o Number of vehicle per unit time (traffic volume) o Vehicle types and speeds o Variation in traffic volumes over time (e.g., Peak Hour) INTRODUCTION TO TRAFFIC ANALYSIS Other dimensions that influenced traffic operations o Traffic control device (i.e., traffic signals, signs and markings) o Types of pavement and geometric design o Selection of the number of lanes INTRODUCTION TO TRAFFIC ANALYSIS Most used numerical dimensions of traffic flow o Speed (km/hr) o Flow/flowrate/volume (veh/hr) o Density (veh/km) FLOW RATE OR VOLUME Flow rate is defined as the number of vehicles passing a point during a specified period of time. SPEED Speed is defined as rate of motion in distance per unit time. When describing traffic stream, two types of speed are used: time mean speed and space mean speed. Space Mean Speed / Harmonic Mean Speed Time Mean Speed / Spot Speed - is used to describe the rate of movement - is simply the arithmetic mean of of a traffic stream within a given section of the speeds of vehicles passing a road. It is the speed based on the average point within a given interval of travel time of vehicles in the stream time. within the section. DENSITY Density is defined as the number of vehicles in a given length of road at an instant point in time. TIME HEADWAY Time headway is defined as the time interval between passage of consecutive vehicles at a specified point on the road with a unit of time per vehicles. SPACING Spacing is the distance between two vehicles measure from the front bumper of a vehicle to that of another. TIME OCCUPANCY It can only be measure, however, if a detector is installed at a specific point on the carriageway. It is defined as the total time of a detector is occupied divided by the total time of observation. RELATIONSHIP OF FLOW, SPEED, AND DENSITY A relationship exists among the three most important traffic variables: flow rate, space mean speed, and density. A dimensional analysis of the units will show that flow rate (veh/hr) is simply the product of density (veh/km) and space mean speed (km/hr), or As mentioned earlier, density is the most difficult variable to measure. It can be obtained indirectly using this relation. Volume-speed-density relations for the inner lane of South Luzon Expressway ` SPEED – DENSITY RELATIONS SPEED – DENSITY RELATIONS VOLUME – DENSITY VOLUME – SPEED RELATION RELATION POISSON DISTRIBUTION MODELS  ORIGIN- DESTINATION SURVEY  Travel surveys are conducted to establish a complete TRAVEL understanding of the travel patterns within the study area. For single projects (such as a highway project), it may be sufficient to use traffic counts on existing roads or (for transit) counts of SURVEY passengers riding the present system. However, to understand why people travel and where they wish to go, origin- destination (O-D) survey data can be useful.  ORIGIN DESTINATION SURVEY ▪ The O-D survey asks questions about each trip that is made on a specific day—such as where the trip begins and ends, the purpose of the trip, the time of day, and the vehicle involved (auto or transit)—and about the person making the trip—age, sex, income, vehicle owner, and so on. Travel Behavior Inventory: Home Interview Survey SOURCE: North Central Texas Council of Governments  ORIGIN DESTINATION SURVEY ▪ The O-D survey may be completed as a home interview, or people may be asked questions while riding the bus or when stopped at a roadside interview station. Sometimes, the information is requested by telephone or by return postcard. O-D surveys are rarely completed in communities where these data have been previously collected. ORIGIN DESTINATION SURVEY O-D data are compared with other sources ▪ to ensure the accuracy and consistency of the results. For example, the number of cars observed crossing one or more bridges might be compared with the number estimated from the surveys. It is also possible to assign trips to the existing network to compare how well the data replicate actual travel. If the screen line crossings are significantly different from those produced by the data, it is possible to adjust in the O-D results so that conformance with the actual conditions is assured. Following the O-D checking procedure, a set of trip tables is prepared that shows the number of trips between each zone in the study area. PARKING STUDIES PARKING STUDIES ▪ Any vehicle traveling on a highway will at one time or another be parked for either a relatively short time or a much longer time, depending on the reason for parking. The provision of parking facilities is therefore an essential element of the highway mode of transportation. The need for parking spaces is usually very great in areas where land uses include business, residential, or commercial activities. In areas of high density, where space is very expensive, the space provided for automobiles usually has to be divided between that allocated for their movement and that allocated for parking them. ▪ Parking studies are therefore used to determine the demand for and the supply of parking facilities in an area, the projection of the demand, and the views of various interest groups on how best to solve the problem. TYPES OF PARKING FACILITIES ▪ On-Street Parking Facilities These are also known as curb facilities. Parking bays are provided alongside the curb on one or both sides of the street. These bays can be unrestricted parking facilities if the duration of parking is unlimited and parking is free, or they can be restricted parking facilities if parking is limited to specific times of the day for a maximum duration. Parking at restricted facilities may or may not be free. Restricted facilities also may be provided for specific purposes, such as to provide handicapped parking or as bus stops or loading bays. ▪ Off-Street Parking Facilities These facilities may be privately or publicly owned; they include surface lots and garages. Self-parking garages require that drivers park their own automobiles; attendant- parking garages maintain personnel to park the automobiles. DEFINITIONS OF PARKING TERMS A space-hour is a unit of parking that defines the use of a single ▪ parking space for a period of 1 hour. ▪ Parking volume is the total number of vehicles that park in a study area during a specific length of time, usually a day. ▪ Parking accumulation is the number of parked vehicles in a study area at any specified time. These data can be plotted as a curve of parking accumulation against time, which shows the variation of the parking accumulation during the day. ▪ The parking load is the area under the accumulation curve between two specific times. It is usually given as the number of space-hours used during the specified period of time. ▪ Parking duration is the length of time a vehicle is parked at a parking bay. When the parking duration is given as an average, it gives an indication of how frequently a parking space becomes available. ▪ Parking turnover is the rate of use of a parking space. It is obtained by dividing the parking volume for a specified period by the number of parking spaces. METHODOLOGY OF PARKING STUDIES A comprehensive parking study usually involves ▪ (1) inventory of existing parking facilities ▪ (2) collection of data on parking accumulation, parking turnover and parking duration ▪ (3) identification of parking generators ▪ (4) collection of information on parking demand. Information on related factors, such as financial, legal, and administrative matters, also may be collected. INVENTORY OF EXISTING PARKING FACILITIES An inventory of existing parking facilities is a detailed listing of the location and all other relevant characteristics of each legal parking facility, private and public, in the study area. The inventory includes both on- and off-street facilities. The relevant characteristics usually listed include the following: ▪ Type and number of parking spaces at each parking facility ▪ Times of operation and limit on duration of parking, if any ▪ Type of ownership (private or public) ▪ Parking fees, if any, and method of collection ▪ Restrictions on use (open or closed to the public) ▪ Other restrictions, if any (such as loading and unloading zones, bus stops, or taxi ranks) ▪ Probable degree of permanency (can the facility be regarded as permanent or is it just a temporary facility?) The information obtained from an inventory of parking facilities is useful both to the traffic engineer and to public agencies, such as zoning commissions and planning departments. The inventory should be updated at regular intervals of about four to five years.  COLLECTION OF PARKING DATA ▪ Accumulation. Accumulation data are obtained by checking the amount of parking during regular intervals on different days of the week. The checks are usually carried out on an hourly or 2-hour basis between 6:00 a.m. and 12 midnight. The selection of the times depends on the operation times of land-use activities that act as parking generators. The information obtained is used to determine hourly variations of parking and peak periods of parking demand.  COLLECTION OF PARKING DATA ▪ Turnover and Duration. Information on turnover and duration is usually obtained by collecting data on a sample of parking spaces in each block. This is done by recording the license plate of the vehicle parked on each parking space in the sample at the ends of fixed intervals during the study period. The length of the fixed intervals depends on the maximum permissible duration. For example, if the maximum permissible duration of parking at a curb face is 1 hour, a suitable interval is every 20 minutes. If the permissible duration is 2 hours, checking every 30 minutes would be appropriate. Turnover is then obtained from the equation IDENTIFICATION OF PARKING GENERATORS ▪ This phase involves identifying parking generators (for example, shopping centers or transit terminals) and locating these on a map of the study area. PARKING DEMAND ▪ Information on parking demand is obtained by interviewing drivers at the various parking facilities listed during the inventory. An effort should be made to interview all drivers using the parking facilities on a typical weekday between 8:00 a.m. and 10:00 p.m. Information sought should include (1) trip origin, (2) purpose of trip, and (3) driver’s destination after parking. The interviewer must also note the location of the parking facility, times of arrival and departure, and the vehicle type. ANALYSIS OF PARKING DATA Analysis of parking data includes summarizing, coding, and interpreting the data so that the relevant information required for decision making can be obtained. The relevant information includes the following: ▪ Number and duration for vehicles legally parked ▪ Number and duration for vehicles illegally parked ▪ Space-hours of demand for parking ▪ Supply of parking facilities The analysis required to obtain information on the first two items is straightforward; it usually involves simple arithmetical and statistical calculations. Data obtained from these items are then used to determine parking space-hours. ANALYSIS OF PARKING DATA The space-hours of demand for parking are obtained from the expression  ANALYSIS OF PARKING DATA The space-hours of supply are obtained from the expression The efficiency factor f is used to correct for time lost in each turnover. It is determined on the basis of the best performance a parking facility is expected to produce. Efficiency factors for curb parking, during highest demand, vary from 78 percent to 96 percent; for surface lots and garages, from 75 percent to 92 percent. Average values of f are 90 percent for curb parking, 80 percent for garages, and 85 percent for surface lots. SAMPLE PROBLEM ▪ The owner of a parking garage located in a CBD has observed that 20% of those wishing to park are turned back every day during the open hours of 8 a.m. to 6 p.m. because of lack of parking spaces. An analysis of data collected at the garage indicates that 60% of those who park are commuters, with an average parking duration of 9 hr., and the remaining are shoppers, whose average parking duration is 2 hr. If 20% of those who cannot park are commuters and the rest are shoppers, and a total of 200 vehicles currently park daily in the garage, determine the number of additional spaces required to meet the excess demand. Assume parking efficiency is 0.90. INTERSECTION DESIGN INTRODUCTION ▪ Intersections play an important role in any network system. They are the points where traffic flow converges and where direction of travel changes. Intersections may be categorized according to shape, type of structure, and type of operation. ▪ Shape ▪ This refers to the configuration of the intersection and would depend largely on the number of legs. Carefully planned and properly designed road networks often lead to intersections with simpler shape having lesser number of legs. Multileg: intersections with more than four legs Three-leg: T or Y Four-leg: normal crossing, oblique, or skewed/staggered Rotary or roundabout INTRODUCTION ▪ Type of Structure ▪ Most of the intersections are either designed as at-grade intersection or grade separation such as flyovers or interchanges. Almost all intersections are initially designed at-grade and are planned to be grade-separated in the future to cope with high traffic volume. The type of grade separation depends largely on the extend of improvement it would provide in terms of easing congestion or reducing traffic accidents. ▪ Type of Operation ▪ Rules and regulations applied to a given intersection depend largely on the type of control which is in operation at that intersection. The main objective is to simplify traffic flow. This is often achieved by reducing the number of conflicts of vehicles. ▪ An intersection operates as unchannelized or channelized, and unsignalized or signalized. Channelization often leads to simplified movements of vehicles as it leads drivers to one conflict at a time. On the other hand, signalization greatly crossing conflicts at the intersection area. BASIC INETRSECTION DESIGN PRINCIPLES ▪ When designing or improving an intersection, it is very important to follow some basic principles to avoid costly mistakes that lead to lack of the intersection’s capacity or to its being accident-prone. ▪ The maximum number of legs should be four. It has been shown that the number of conflicts increases exponentially as the number of intersection legs increases. ▪ Staggered intersections should be avoided. Due to proximity of the two side roads, inadequate length of storage lane for left vehicles often causes blockage of the through traffic. ▪ Main traffic flow should be near straight as possible. Sharp turns, such as left and right turns, cause unnecessary reduction in traffic speeds. BASIC INETRSECTION DESIGN PRINCIPLES ▪ Roads should not intersect at a small angle. Oblique intersections pose potential hazards and cause high severity of accidents due to the almost head-to-head collision of vehicles. An angle of 60 degrees must be considered as the minimum. ▪ Two intersections should be as far as possible from each other. In addition to the reason cited in (2), adequate weaving sections may not be provided. As a rule of thumb, the distance between the two intersections must be distance = design speed (kph) x number of lanes x 2 For instance, if the prevailing speed is 30 kph and three are four lanes in one direction, the distance between the two intersections must be at least 30 x 4 x 2 = 240 m. INTERSECTION DESIGN ELEMENTS ▪ Turning geometry ▪ The principal purpose of an intersection is to provide change in the direction of travel. As a vehicle approaches an intersection, the driver has to decide whether to go straight or to turn to left or right. For turning movements, a number of turning geometries may be considered, the most direct of which is highly preferred. Configurations of turning geometries are shown below: Direct Indirect Semidirect INTERSECTION DESIGN ELEMENTS ▪ Turning geometry ▪ These types of turning movements may be found as elements of the interchanges shown below: INTERSECTION DESIGN ELEMENTS ▪ Turning geometry ▪ The different elements of an intersection are shown in Figure 5.1. The adequacy in design of each element must be carefully checked, considering traffic flow and availability of right of way. For the left turn storage bay, the recommended length is = 2 x no. of left turners in one cycle x spacing in queue. The spacing in queue may range from 6.0 m to 7.0 m, depending on the type of vehicles using the intersection. A factor of 2 is used to take into account randomness of traffic flow. INTERSECTION DESIGN ELEMENTS ▪ Turning geometry The number of lanes for through, right, and left turn vehicles would depend on traffic volume, saturation flow rates, a simple circular curve may be adequate for the design of the intersection corners and the turning roadway. However, simple curve may not be enough for large intersections. The most common type of geometry for these intersections is the three- Three-centered curve centered curve as shown in the figure. METHODS OF CONTROL OF INTERSECTIONS ▪ Conflicts often occur at intersections. The more the number of legs an intersection has, the more the number of conflicts it has. Conflict diagrams for three-leg and four leg intersections METHODS OF CONTROL OF INTERSECTIONS ▪ Conflicts may be classified as merging, diverging, or crossing conflicts. The table below gives a summary of these conflicts. The total number increases exponentially with the number of legs of the intersection. Most of the problems at intersections, like congestion and accidents, are caused by crossing conflicts. Types and number of conflicts METHODS OF CONTROL OF INTERSECTIONS ▪ Conflicts may be classified as merging, diverging, or crossing conflicts. The table below gives a summary of these conflicts. The total number increases exponentially with the number of legs of the intersection. Most of the problems at intersections, like congestion and accidents, are caused by crossing conflicts. Types and number of conflicts METHODS OF CONTROL OF INTERSECTIONS ▪ Depending on the traffic volume using the intersection and the severity of conflicts, intersection control may fall under any of the following categories: a. Unsignalized b. Signalized c. Grade separation  Conflict diagrams for different types of control for a four-leg intersections Comparison of types of control for a four-leg intersection  Grade separation or interchanges ANALYSIS OF UNSIGNALIZED INTERSECTIONS ▪ A large number of intersections all over the country are still without traffic signals. It is therefore necessary to have a means of analysis of the performance of this type of intersection so as to find appropriate measures to minimized congestion and reduce the occurrence of traffic accidents. ▪ The method calculates the maximum flow in any given minor road traffic stream. It is them compared with the existing traffic flow to estimate the reserve capacity. The probable delay and level of service are determined based on this reserve capacity. ANALYSIS OF UNSIGNALIZED INTERSECTIONS ▪ Consider the four-leg intersection below. Minor movements are movements coming from the side road or minor road. In addition, the left turn movements from the major road are also considered minor. But in terms of hierarchy, these left turn movements have higher priority than the movements coming from the minor road. The method requires that the traffic movements be dealt with in the following order: a. Right turns into the major road b. Left turns off the major road c. Traffic crossing the major road d. Left turns into the major road ANALYSIS OF UNSIGNALIZED INTERSECTIONS ▪ Structure of Major Road Traffic ANALYSIS OF UNSIGNALIZED INTERSECTIONS ▪ Critical Gap ▪ The source of capacity of the minor road flows is the available gaps between major road flows. The critical gap used to describe the minimum gaps needed by drivers of minor road vehicles. Values of critical gaps are given in the table for different vehicle maneuvers, speed limits, and highway type. ANALYSIS OF UNSIGNALIZED INTERSECTIONS ▪ Capacity ▪ In the estimation of capacity of the minor road flow, the basic capacity is initially determined. Based on the major road flows given by Mh, and values of critical gap tg, the value of the basic capacity Mno is read from the graph shown in the figure. The basic capacity is the maximum minor road flow, assuming that the following conditions are true: a. The traffic on the major road Mh does not block the major road. b. A turning lane is provided for the exclusive use of the minor road traffic stream. ANALYSIS OF UNSIGNALIZED INTERSECTIONS ▪ Capacity ▪ If these conditions are not met, correction factors have to be implied based on the following considerations: ▪ Congestion on the major road Reduction factor due to possible congestion of movement of major road ANALYSIS OF UNSIGNALIZED INTERSECTIONS ▪ Capacity ▪ If these conditions are not met, correction factors have to be implied based on the following considerations: ▪ Shared lanes where Mn - capacity of all streams using the shared lane Mna, Mnb, Mnc - capacity of individual streams a, b, c - contribution of the individual streams to the total volume using the shared lane. ANALYSIS OF UNSIGNALIZED INTERSECTIONS ▪ Capacity ▪ If these conditions are not met, correction factors have to be implied based on the following considerations: ▪ Passenger Car Equivalents. Since the calculated capacity is in pcu/hr, the existing flow has to be converted to the same unit. PCU values of different vehicle types ANALYSIS OF UNSIGNALIZED INTERSECTIONS ▪ Capacity ▪ If these conditions are not met, correction factors have to be implied based on the following considerations: ▪ Reserve Capacity SAMPLE PROBLEM ▪ Consider the channelized T-intersection with the minor road controlled by YIELD sign. There is no prevailing speed limit. Evaluate the performance of the unsignalized intersection. SAMPLE PROBLEM ▪ Evaluate the unsignalized intersection with the two minor road approaches controlled by STOP signs. The traffic volumes shown are in vehicles per hour (vph). There is no prevailing speed limit. ANALYSIS AND DESIGN OF ROUNDABOUTS OR ROTONDAS ▪ The elements of the roundabout are shown in figure 5.10. Roundabout with large central island are known to operate better as they provide ample weaving sections that normally dictate the capacity of the intersection. design elements of a roundabout ANALYSIS AND DESIGN OF ROUNDABOUTS OR ROTONDAS ▪ The elements of the roundabout are shown in figure 5.10. Roundabout with large central island are known to operate better as they provide ample weaving sections that normally dictate the capacity of the intersection. design elements of a roundabout ANALYSIS AND DESIGN OF ROUNDABOUTS OR ROTONDAS ▪ For the analysis and design of roundabouts, British practice will be discussed in this section (Salter 1976). The focus is the capacity of the weaving section, which is given by the equation where w – the width of the weaving section in meters e – the average width of entries to the weaving section in meters; l – the length of the weaving section Weaving section ANALYSIS AND DESIGN OF ROUNDABOUTS OR ROTONDAS ▪ The capacity formula is valid under the following conditions, although there is no reason to believe that the formula does not hold if any variable lies a little outside the values given: ▪ The design of roundabouts is usually done by trial and error. Weaving width and length are assumed and then checked for adequacy of capacity. As roundabouts have a tendency to lock when overloaded, it is important that they have adequate reserve capacity. SAMPLE PROBLEM ▪ Consider the Y-junction with the given traffic volume to be designed as a roundabout. THANK YOU!