This Document Contains Chapters 7 to 8 Chapter 7 Intersection Design 7-1 Briefly describe the different principles involved in the design of at-grade intersections. The fundamental objective in the design of at-grade intersections is to minimize the severity of potential conflicts both among different streams of traffic and between pedestrians and turning vehicles while facilitating smooth traffic flow. The design should therefore incorporate the operating characteristics of both the vehicles and pedestrians using the intersection. The design of an at-grade intersection involves the design of the alignment (horizontal and vertical) of the intersecting roadways, the design of a suitable channeling system, the determination of the minimum required widths of turning roadways when traffic is expected to make turns at speeds higher than 15 mi/h, and the assurance that sight distances are adequate for the type of control at the intersection. 7-2 Describe the different types of at-grade intersections. Also give an example of an appropriate location for the use of each type. The basic types of at-grade intersections are T (three-leg) intersections, four-leg intersections, multi-leg intersections consisting of 5 or more approaches, and traffic circles. A simple T intersection is suitable for intersections of minor roads. Four-leg intersections are used mainly at locations where minor or local roads crossed, although it can be used where a minor road crosses a major highway. Multi-leg intersections should be avoided whenever possible. When a multi-leg intersection exists, one leg should be realigned, if possible, into a T intersection with the minor road, at a distance far enough away from the 4-leg intersection to allow for independent operation of the intersections. Traffic circles force traffic to use the intersection in a circular pattern, thereby transforming crossing conflicts into merging and diverging conflicts. The neighborhood traffic circle is placed for traffic calming purposes on local streets in residential areas to reduce travel speeds and cut-through traffic. 7-3 Describe the different types of traffic circles indicating under what conditions you will recommend the use of each. According to a Federal Highway Administration report, traffic circles can be grouped into three categories: rotaries, neighborhood traffic circles, and roundabouts. A rotary could be where maintaining high speed is important and a large amount of right-of-way is available to accommodate the large radius required for a rotary. A neighborhood traffic circle may be suitable at the intersection of local streets in a low-speed situation where there is interest in traffic calming measures; typically stop control or no control is used on the approaches. A roundabout may be suitable for situations intermediate to the two described above. 7-4 What are the key defining characteristics of roundabouts that distinguish them from other traffic circles? Roundabouts, as opposed to other types of traffic circles, have yield control used on the approaches, and the roundabout design speed typically does not exceed 30 miles per hour. Conflicting traffic movements are separated by pavement markings or channelizing islands such that vehicles are given detailed guidance as to proper place within the roundabout. Parking is prohibited within the roundabout to maximize capacity. 7-5 What are the main functions of channelization at an at-grade intersection? Channelization is intended to separate conflicting traffic movements into defined paths to facilitate the safe and orderly movements of both vehicles and pedestrians. This can accomplish many things, including: Direct the paths of vehicles so that not more than two paths cross at one point Control the merging, diverging, or crossing angle of vehicles Decrease vehicle wander and the area of conflict among vehicles by reducing the amount of paved area Provide a clear indication of the proper path for different movements Give priority to the predominant movements Provide pedestrian refuge Provide separate storage lanes for turning vehicles, thereby creating space away from the path of through vehicles for turning vehicles to wait Provide space for traffic control devices so that they can be readily seen Control prohibited turns Separate different traffic movements at signalized intersections with multiplephase signals 7-6 Discuss the fundamental general principles that should be used in designing a channelized at-grade intersection. There are several fundamental principles considered in channelized intersection design. Regarding guidance to the motorist, motorists should not be required to make more than one decision at a time. Merging and weaving areas should be as long as possible, but other areas of conflict between vehicles should be reduced to a minimum. Crossing traffic streams that do not weave or merge should intersect at 90°, although a range of 60-120° is acceptable; adequate sight distance is provided regardless of angle. Sharp reverse curves and turning paths greater than 90° should be avoided. Separate space should be provided for turning vehicles so that they do not interfere with the movement of through vehicles. Prohibited turning movements should be blocked with channelized islands wherever possible. Finally, the location of essential traffic control devices should be considered in the design process. 7-7 Describe the different types of islands used in channelizing at-grade intersections indicating the principal function of each type. Traffic islands can be divided into three categories: channelizing, divisional, and refuge. Channelizing islands control and direct traffic in merging and diverging situations to guide motorists into the correct lanes for their movements at the intersection. Divisional islands to divide traffic in opposing directions at intersections to provide clear and separate paths for each direction of traffic. Refuge islands provide a stopping place out of the path of motorists for pedestrians crossing wide intersections. 7-8 Figure 7.26a illustrates a three-leg intersection of State Route 150 and State Route 30. Both roads carry relatively low traffic with most of the traffic oriented along State Route 150. The layout of the intersection, coupled with the high-speed traffic on State Route 150, have made this intersection a hazardous location. Drivers on State Route 30 tend to violate the stop sign at the intersection because of the mild turn onto westbound State Route 150, and they also experience difficulty in seeing the high-speed vehicles approaching from the left on State Route 150. Design a new layout for the intersection to eliminate these difficulties for the volumes shown in Figure 7.26b. Design vehicle is a passenger car. The intersection shown in the figure is a T-intersection with the two roadways intersecting at an angle less than 90 degrees. This creates a condition where the right turning vehicles from the minor road (SR 30) tend to violate the stop sign because the intersection configuration causes that movement to resemble a through movement. The intersection should be realigned so that the minor road (SR 30) intersects SR 150 at a right angle. This will cause the minor road rightturning vehicles to make a much more defined turn, forcing them to slow down and stop. The realignment will clearly indicate that SR 30 is the minor road, and by creating a perpendicular intersection, drivers turning either way from the minor road will have improved sight lines along the major road. The following figures show the existing intersection and proposed realignment, respectively. 7-9 Figure 7.27a shows the staggered unsignalized intersection of Patton Avenue and Goree Street. The distance between the T intersections is about 160 ft. The general layout and striping of the lanes at this intersection result in confusion to drivers and create multiple conflicts. Design an improved layout for the intersection for the traffic volumes shown in Figure 7.27b. Design vehicle is a passenger car. The staggered intersection of Patton Avenue and Goree Street will tend to cause confusion among drivers who expect to find cross streets intersecting perpendicularly, without the stagger as shown. The east leg of Patton Avenue should be realigned to produce a 4-leg intersection. Doing so should reduce the number of turning accidents, which appear to be the most common type of accident at this location. The high number of rear end accidents should also be reduced since motorist confusion will be reduced. 7-10 An existing intersection consists of a turning movement of 75 degrees, and the radius of the edge of pavement is 60 ft. The turning radius has no entry taper. What is the largest design vehicle that is accommodated at the intersection? What improvements would you recommend be made if this movement is to accommodate a WB-67 design vehicle? Table 7.2 shows the minimum edge of pavement design for turns at intersections for simple curves and simple curves with tapers. It can be observed that for an angle of turn of 75º and simple curve radius of 60 ft, the largest design vehicle that can be accommodated at the intersection is a SU-30. Table 7.2 also indicates that it is not feasible to have simple curves for large trucks when the angle of turn is 75 degrees or greater. One option to accommodate a WB-67 design vehicle at this intersection is to add a 20:1 taper, with offset of 4.5 ft, and the simple curve radius should be increased to at least 145 ft. Table 7.3 shows the minimum edge of pavement design for turns at intersections for symmetric and asymmetric 3-centered compound curves. Another option to accommodate the WB-67design vehicle is to change the simple curve to a 3-centered compound curve, which will require curve radii of 420-75420 ft with 10ft offset for symmetric curves, or 200-80-600 ft with 1.0 and 10.0 offsets for asymmetric curves. 7-11 An intersection is being designed for two roads that intersect at approximately 60 degrees. For an SU-30 design vehicle, recommend a design for pavement edge for the right turn movement for the 60-degree turns and the 120-degree turns. 60º turn angle: To accommodate the SU-30 design vehicle, a simple curve can be designed for a 60º angle of turn. According to Table 2, the simple curve does not require a taper and the radius should be at least 60 ft. Table 3 shows that a compound curve is not indicated for the 60º turn angle if only a SU-30 design vehicle should be accommodated. 120º turn angle: A simple curve can also be designed to accommodate the SU-30 design vehicle at the intersection with a 120º turn angle, and Table 2 indicates that it requires a 10:1 taper, with 3.0 ft offset, and the radius of the curve should be at least 30 ft. For the 120º turn angle, a symmetric 3-centered compound curve can also be designed. Table 3 shows that the radii should be 100-30-100 ft, with symmetric offset of 3.0 ft, to accommodate the SU-30 design vehicle. An asymmetric 3centered compound curve is not indicated for this intersection. 7-12 A ramp from an expressway with a design speed of 30 mi/h connects with a local road, forming a T intersection. An additional lane is provided on the local road to allow vehicles from the ramp to turn right onto the local road without stopping. The turning roadway has stabilized shoulders on both sides and will provide for a onelane, one-way operation with no provision for passing a stalled vehicle. Determine the width of the turning roadway if the design vehicle is a single-unit truck. Use 0.08 for superelevation. The first step is to determine the minimum radius of curvature for the ramp, using Equation 3.34. Coefficient of side friction can be seen in Table 3.3: R = u2 / 15(e + fs) R = (30)2/15(0.08 + 0.20) R = 214.29 ft From Table 7.5, determine appropriate pavement width, W, knowing that: Operational requirements: Case I – one-lane, one-way operation, no provision for passing a stalled vehicle Traffic conditions: Type B – SU is the design vehicle Edge of pavement treatment: Stabilized shoulder Using the table, interpolating for R = 214 ft, W = 15 ft Therefore, the width of the turning roadway should be 15 ft. 7-13 Determine the required width of the turning roadway in Problem 7-12 for a twolane operation with barrier curbs on both sides. The first step is to determine the minimum radius of curvature for the ramp, using Equation 3.34. Coefficient of side friction can be seen in Table 3.3: R = u2 / 15(e + fs) R = (30)2/15(0.08 + 0.20) R = 214.29 ft From Table 7.5, determine appropriate pavement width, W, knowing that: Operational requirements: Case III – two-lane operation, either oneway or two-way Traffic conditions: Type B – SU is the design vehicle Edge of pavement treatment: barrier curb on both sides Using the table, interpolating for R = 214 ft, W = 28 ft: for the edge of pavement treatment specified, add 2 ft. Therefore, the width of the turning roadway should be 30 ft. 7-14 Repeat Problem 7-12 for a one-lane, one-way operation with provision for passing a stalled vehicle. The first step is to determine the minimum radius of curvature for the ramp, using Equation 3.34. Coefficient of side friction can be seen in Table 3.3: R = u2 / 15(e + fs) R = (30)2/15(0.08 + 0.20) R = 214.29 ft From Table 7.5, determine appropriate pavement width, W, knowing that: Operational requirements: Case II – one-lane, one-way operation, with provision for passing a stalled vehicle Traffic conditions: Type B – SU is the design vehicle Edge of pavement treatment: Stabilized shoulder Using the table, interpolating for R = 214 ft, W = 20 ft Therefore, the width of the turning roadway should be 20 ft. 7-15 A four-leg intersection with no traffic control is formed by two 2-lane roads with the speed limits on the minor and major roads being 25 and 40 mi/hr, respectively. If the roads cross at 90° and a building is to be located at a distance of 50 ft from the centerline of the nearest lane on the minor road, determine the minimum distance at which the building should be located from the centerline of the outside lane of the major road so that adequate sight distances are provided. Grades are approximately level. To ensure sufficient sight distances for each intersection approach, a line of sight must be provided that allows for sufficient time for a driver to perceive and react to a vehicle on the crossing approach. The required distance (offset of building from roadway) can be found using Table 7.7 and Equation 7.4. db = a da (d ba − ) Given: b = 50 ft From Table 7.7: da = 195 ft (major approach) db = 115 ft (minor approach) 115 / 195 = a / (195 – 50) a = 85.51 ft The distance between the centerline of the outside lane on the major road and the building should be 86 ft. 7-16 A major roadway with a speed limit of 45 mi/h has an intersection that has no intersection control with a minor roadway at a right angle. A building is located next to the intersection at a distance of 60 ft from the center of the near lane on the major roadway and 40 ft from the center of the near lane on the minor roadway. At what speed for the minor roadway is the intersection safe? Grades are approximately level. First, determine the distance on the minor road at which the driver first sees traffic on the intersecting road. It is known that: Speed limit of major road = 45 mi/h da = 220 ft (from Table 7.7) a = 60 ft b = 40 ft From Equation 7.4: db a = da (d ba − ) db / 220 = 60 / (220 – 40) db = 73.33 ft Table 7.7 shows that the maximum allowable speed on the minor road for db = 73.33 ft is 15 mi/h. No correction is required for the approach grade as it is approximately level. 7-17 What are the main deficiencies of multi-leg intersections? Using a suitable diagram show how you will correct for these deficiencies. The main deficiencies of multi-leg intersections are associated with sight distance and clear assignment of right-of-way. Intersections with more than four approaches can be corrected by relocating one or more legs to intersect with another approach away from the main intersection. 7-18 A two-lane minor road intersects a two-lane major road at 90° forming a four-leg intersection with traffic on the minor road controlled by a yield sign. A building is located 110 ft from the centerline of the outside lane of the major road and 40 ft from the centerline of the nearest lane of the minor road. Determine the maximum speed that can be allowed on the minor road if the speed limit on the major road is 40 mi/hr. Grades are approximately level. Given: = 110 ft = 40 ft Table 7.9 discusses the different cases when different speed limits are applied on the minor road. When vminor = 15 mph, db = 75 ft < a = 110 ft. Hence, there is no restriction on the speed limit on the major road. When vminor = 20 mph, db = 100 ft a = 110 ft, and tg =6.5 sec. From Equation 7.7, the sight distance along the major road should be da = 1.47vmajor gt = 1.47× ×40 6.5 = 382.2 ft At site, the maximum sight distance along the major road can be computed using Equation 7.4 : da mzx, b = db (d ab − ) da, max = (130)(40)/(130-110) = 260 ft 5. The crash criterion is satisfied. This intersection has traffic volumes on all the approaches approximately equal, 230 veh/h for major roads and 220 veh/h for minor roads. As it can be observed on step 2 of this problem, the minor road traffic volume criterion is satisfied. Also, there are more than 5 crashes in a 12-month period that could be avoided with a multiway stop control, so the crash criterion is satisfied. The installation of a multiway stop control is supported. 8-4 A two-phase signal system is installed at the intersection described in Problem 8-1, with channelized left-turn lanes and shared through and right-turn lanes. Using a suitable diagram, determine the possible conflict points. Indicate the phasing system used. The recommended phasing for the intersection configuration shown below consists of two phases: one for westbound traffic (left and through, one for westbound traffic (through) and eastbound traffic (through-right), and one for northbound traffic (left and through-right). Phase 1:
Diverging Conflict Merging Conflict Crossing Conflict
Phase 2: 8-5 Using appropriate diagrams, determine the possible conflict points on a four-leg signalized intersection for a two-phase system. Assume no turn on red.
Diverging Conflict Merging Conflict Crossing Conflict
(a) Phase 1:
8-6 Repeat Problem 8-5 for the following phasing systems: Four-phase with separate phases for left turns Four-phase with separate phase for each approach Four-phase with separate phases for left turns Phase 1: Phase 2: Phase 3: Phase 4: Four-phase with separate phase for each approach
Diverging Conflict Merging Conflict Crossing Conflict
Phase 1: Phase 2: Phase 3: 8-7 Determine whether this intersection for which data are provided below meets MUTCD Warrants 1, 2, and/or 3 for signalization. These data were collected using pneumatic tubes and the data were binned in 60-minute increments (15-minute data are not available). All approaches have one lane entering the intersection. For any warrants that are met, explain how the warrant was met (rules applied, hours in which the volume criteria were met, etc.). The major street speed limit is 45 mph, and the minor street speed limit is 25 mph. The minor street has only one approach (i.e. it is a “T” intersection.)
Hour Beginning Major Street Volume Minor Street Volume Hour Beginning Major Street Volume Minor Street Volume
0 25 3 12 537 112
1 28 2 13 558 82
2 31 1 14 736 91
3 59 2 15 785 99
4 270 27 16 689 81
5 714 115 17 546 71
6 774 159 18 396 60
7 460 151 19 264 42
8 479 123 20 193 29
9 430 93 21 149 21
10 508 98 22 84 15
11 455 85 23 36 8
Warrant 1: Eight-hour vehicular volume Condition A: Table 8.1: Major street: 1 lane; Minor street: 1 lane Vehicles per hour on major street: 350 (using the 70% rule, as the major street speed exceeds 40 mph) Vehicles per hour on minor street: 105 (using the 70% rule, as the major street speed exceeds 40 mph) Only 5 hours satisfy Condition A: 5, 6, 7, 8, and 12. - Condition not met. Condition B: Table 8.2: Major street: 1 lane; Minor street: 1 lane Vehicles per hour on major street: 525 (using the 70% rule, as the major street speed exceeds 40 mph) Vehicles per hour on minor street: 53 (using the 70% rule, as the major street speed exceeds 40 mph) 8 hours satisfy Condition A: 5, 6, 12, 13, 14, 15, 16, and 17. Condition met. Condition B is satisfied; therefore, Warrant 1 is satisfied. Warrant 2: Four- hour vehicular volume Figure 8.7 (70 percent condition as speed exceeds 40 mph) 1 lane and 1 lane 75 vph is the lower threshold volume for a minor-street approach with one lane Only one hour, Hour 6, falls above the line. Warrant 2 is not satisfied. Warrant 3: Peak hour Condition A: no information about delay Condition B: Figure 8.6 1 lane and 1 lane 100 vph is the lower threshold volume for a minor-street approach with one lane None of the hours fall above the line. Warrant 3 is not satisfied. 8-8 A railroad track is crossed by a minor street that intersects a major street that runs parallel to the railroad track. There is a concern about queuing of minor street traffic, controlled by a STOP sign at the intersection with the major street, backward onto the track. Determine whether this intersection meets MUTCD Warrant 9, given the following data: Major street peak hour volume (two-way): 320 veh/h Minor street peak hour volume (on the one-lane approach that crosses the tracks): 120 veh/h; 6% of which are tractor-trailer trucks; bus traffic is negligible Rail traffic is an average of 8 trains per day distributed in a scattered manner throughout the day The distance on the minor street between the nearest rail of the track and the STOP line at the intersection with the major street is 96 ft. Condition A: distance on the minor street between the nearest rail of the track and the STOP line at the intersection with the major street is 96 ft (G+Y)1 , Gp3 > (G+Y)3 , and Gp4 > (G+Y)4 , the allocated sum of green and yellow time should be 30.0 seconds for phase 1, 33.4 seconds for phase 3, and 33.4 seconds for phase 4. Times are typically rounded up to the next whole seconds; therefore, sum of green and yellow times are: G1 = 30 s; G2 = 37 s; G3 = 34 s; G4 = 34 s, resulting in a total cycle length of C = (30 + 37 + 34 + 34) + (4)(1.5) = 141 seconds. By increasing saturation flow rates by 10%, the recommended cycle length decreased by 14.5% (from 165 seconds to 141 seconds). 8-18 Repeat Problem 8-16 using pedestrian flow rates that are 20% higher. What effect does this have on cycle length? Assume the following saturation flows:
Through lanes: 1600 veh/h/ln
Through-right lanes: 1400 veh/h/ln
Left lanes: 1000 veh/h/ln
Left-through lanes: 1200 veh/h/ln
Left-through-right lanes: 1100 veh/h/ln
Step 1: Calculate equivalent hourly flows
Approach (Width) North (56 ft) South (56 ft) East (68 ft) West (68 ft)
Peak hour approach volumes
Left turn 140 77 177 141
Through movement 442 393 593 543
Right turn 147 142 178 187
Step 2: Assume an intersection configuration, assign lane groups, and determine critical volumes. In this case, each approach was assumed to have one dedicated left-turn lane, one through lane, and one through-right lane.
Approach (Width) North (56 ft) South (56 ft) East (68 ft) West (68 ft)
Peak hour approach volumes
Left 140 77 177 141
Through-right 589 535 771 730
Step 3: Assume a phasing scheme and determine Yi , sum of critical ratios Assume four phases as follows:
Phase 1: E-W left Phase 2: E-W thru Phase 3: N-S left Phase 4: N-S thru
qij 177 771 140 589
sij 1000 3000 1000 3000
Yi = qij/sij 0.177 0.257 0.140 0.196
Sum Yi = 0.770 Step 4: Calculate lost time per cycle, using Equation 8.8 Assume lost time per phase due to acceleration and deceleration at phase changes is 3.5 seconds and that an all-red interval of 1.5 seconds is provided at each phase. Total lost time, L = 20 sec. Step 5: Calculate cycle length, using Equation 8.6 C= 1.5 L+ 5 = ((1.5)(20)+5)/(1-0.770) = 152.2 seconds 1−Yi Use C = 155 seconds Step 6: Allocate green times Allocated times are for green and yellow indications; appropriate length of yellow interval can be subtracted from the total to give green times. Total effective green time, Gte = C – L =135 seconds (G+Y)1 = (0.177/0.770)(135) + 3.5 = 34.5 seconds (G+Y)2 = (0.257/0.770)(135) + 3.5 = 48.5 seconds (G+Y)3 = (0.140/0.770)(135) + 3.5 = 28.0 seconds (G+Y)4 = (0.196/0.770)(135) + 3.5 = 37.9 seconds Step 7: Ensure that green time required for pedestrian movement is provided, using Equation 8.12. Gp1 = 3.2 + (56/3.5) + (0.27)(1440/3600)(155) = 35.9 seconds Gp2 = 3.2 + (56/3.5) + (0.27)(1440/3600)(155) = 35.9 seconds Gp3 = 3.2 + (68/3.5) + (0.27)(1440/3600)(155) = 39.4 seconds Gp4 = 3.2 + (68/3.5) + (0.27)(1440/3600)(155) = 39.4 seconds Since Gp1 > (G+Y)1 , Gp3 > (G+Y)3 , and Gp4 > (G+Y)4 , the allocated sum of green and yellow time should be 35.9 seconds for phase 1, 39.4 seconds for phase 3, and 39.4 seconds for phase 4. Times are typically rounded up to the next whole seconds; therefore, sum of green and yellow times are: G1 = 36 s; G2 = 49 s; G3 = 40 s; G4 = 40 s, resulting in a total cycle length of C = (36 + 49 + 40 + 40) + (4)(1.5) = 171 seconds. By increasing conflicting pedestrian volumes by 20%, the recommended cycle length increased by 3.6% (from 165 seconds to 171 seconds). 8-19 Repeat Problem 8-16 using the HCM method and a critical v/c of 0.9. Step 1: Determine critical ratios From Problem 8-16: Phase 1: Y1 = 0.177 Phase 2: Y2 = 0.257 Phase 3: Y3 = 0.140 Phase 4: Y4 = 0.196 Σ ((q/s)i = Σ Yi = 0.770 Step 2: Determine cycle length (using Equation 8.15) Xc = 0.9 (critical v/c ratio) L = 20 sec (lost time, from Problem 8-5) Xc = Σ ((q/s)i (C/(C-L)) 0.9 = 0.770 (C/(C-20)) 1.169 C – 23.38 = C C = 138.32 sec; use 140 sec Step 3: Determine phase lengths Allocated times are for green and yellow indications; appropriate length of yellow interval can be subtracted from the total to give green times. Total effective green time, Gte = C – L =120 seconds (G+Y)1 = (0.177/0.770)(120) + 3.5 = 31.1 seconds (G+Y)2 = (0.257/0.770)(120) + 3.5 = 43.5 seconds (G+Y)3 = (0.140/0.770)(120) + 3.5 = 25.3 seconds (G+Y)4 = (0.196/0.770)(120) + 3.5 = 34.1 seconds Step 4: Ensure that green time required for pedestrian movement is provided, using Equation 8.12. Gp1 = 3.2 + (56/3.5) + (0.27)(1200/3600)(140) = 31.8 seconds Gp2 = 3.2 + (56/3.5) + (0.27)(1200/3600)(140) = 31.8 seconds Gp3 = 3.2 + (68/3.5) + (0.27)(1200/3600)(140) = 35.2 seconds Gp4 = 3.2 + (68/3.5) + (0.27)(1200/3600)(140) = 35.2 seconds Since Gp1 > (G+Y)1 , Gp3 > (G+Y)3 , and Gp4 > (G+Y)4 , the allocated sum of green and yellow time should be 31.8 seconds for phase 1, 35.2 seconds for phase 3, and 35.2 seconds for phase 4. Times are typically rounded up to the next whole seconds; therefore, sum of green and yellow times are: G1 = 32 s; G2 = 44 s; G3 = 36 s; G4 = 36 s, resulting in a total cycle length of C = (32 + 44 + 36 + 36) + (4)(1.5) = 154 seconds. 8-20 Using the results for Problems 8-16 and 8-19, compare the two different approaches used for computing cycle length. The HCM method (used in Problem 8-19) is less computationally intensive than Webster method (used in Problem 8-16). The HCM method allows for a desired v/c ratio to be a determining factor in cycle length. Webster’s method yielded a cycle length of 11 seconds longer than the HCM method. 8-21 Briefly describe the different ways the traffic signals at the intersection of an arterial route could be coordinated, stating under what conditions you would use each of them. Traffic signals can be coordinated by several methods: simultaneous system, alternate system, and progressive system. In a simultaneous system, all signals have the same cycle length and are in the green phase for the arterial at the same time. This system works best when intersections are approximately the same distance apart. In an alternate system, intersections are formed into groups where successive groups alternate green phases. The alternate system works best when the intersections within a group are at equal distance from each other. In a progressive system an offset is introduced between the start of green for the arterial at one intersection and the start of green for the arterial at the succeeding intersection. This offset is based on the distance between intersections and the speed of traffic. This method best accommodates variable spacing between intersections and heavy directional flows. 8-22 You have been asked to design a simultaneous traffic signal system for six intersections on a suburban arterial. The distances between consecutive intersections are: Intersection A to Intersection B – 3,800 ft Intersection B to Intersection C – 4,000 ft Intersection C to Intersection D – 3,900 ft Intersection D to Intersection E – 3,850 ft Intersection E to Intersection F – 3,950 ft Suitable cycle lengths for the intersections are: Intersection A – 60 sec Intersection B – 55 sec Intersection C – 65 sec Intersection D – 60 sec Intersection E – 55 sec Intersection F – 60 sec If an appropriate progression speed for the arterial is 50 mph, what cycle length would you use if the signals are not actuated? Give a reason for your choice. For this arterial, the average distance between intersections is 3,900 ft. By rearranging Equation 8.19, an appropriate cycle length can be determined: = 1.47 = 1.47 × 50 = 53.06 Since the cycle lengths for the individual signals would be 55 or 60 seconds, and the computed value for progression is 53.06 seconds, an appropriate cycle length would be 55 seconds. 8-23 In Problem 8-22, if conditions at intersection C require that a cycle length of 65 sec be maintained through the corridor, what will be a suitable progression speed? Equation 8.19 can be used to determine an appropriate progression speed if the cycle lengths for the system must be 65 seconds. u= X = 3900 = 40.9 mi/h 1.47C (1.47)(65) The computed value of 40.9 mi/h is approximately equal to 40 mi/h, therefore, a progression speed of 40 mph is recommended. 8-24 In Problem 8-22, an intersection located midway between intersections B and C is to be signalized. Recommend a cycle length and progression speed for the corridor that includes the new signal if the signals are not actuated. If we have a new intersection N, the distances between consecutive intersections will be: Intersection A to Intersection B – 3,800 ft - Intersection B to Intersection N – 2,000 ft - Intersection N to Intersection C – 2,000 ft - Intersection C to Intersection D – 3,900 ft Intersection D to Intersection E – 3,850 ft Intersection E to Intersection F – 3,950 ft For this arterial, the average distance between intersections is 3,250 ft. Equation 8.19 can be used to determine an appropriate progression speed if the cycle lengths for the system must be 55, 60, or 65 seconds. = 1.47 C = 55 seconds: = 1.47 × 55 = 40.20 /ℎ C = 60 seconds: = 1.47 × 60 = 36.85 /ℎ C = 65 seconds: = 1.47 × 65 = 34.01 /ℎ 8-25 Describe the effects of the three left-turn treatments at signalized intersections on saturation flow rate and geometric design. Left-turn vehicles at signalized intersections can proceed under one of three signal conditions: permitted, protected, and protected/permissive turning movements. Permitted turning movements are those made within gaps of an opposing traffic stream or through a conflicting pedestrian flow and should yield to conflicting traffic and pedestrian movements. Permitted operation is primarily used when traffic is light to moderate and sight distance is adequate. This mode can have an adverse affect on safety in some situations, such as when the left-turn driver's view of conflicting traffic is restricted or when adequate gaps in traffic are not present. Protected turns are those turns protected from any conflicts with vehicles in an opposing stream or pedestrians on a conflicting crosswalk. All conflicting movements must therefore yield to protected turns. A permitted turn takes more time than a similar protected turn and will use more of the available green time. This operation provides for efficient left-turn movement service; however, the added left-turn phase increases the lost time within the cycle length and may increase delay to the other movements. An exclusive left-turn lane is typically provided with this phasing. The left-turn phase is indicated by a green arrow signal indication. This type operation is recognized to provide the safest left-turn operation. Protected-Permitted is a combination of the protected and permissive conditions, in which vehicles are first allowed to make left turns under the protected condition and then allowed to make left turns under the permissive condition. This mode provides for efficient left-turn movement service, often without causing a significant increase in delay to other movements. This mode also tends to provide a relatively safe left-turn operation, provided that adequate sight distance is available and turns during the permissive component can be safely completed. The determination of the specific treatment at a location depends on the transportation jurisdiction as guidelines vary from one jurisdiction to another. A chart proposed in the FHWA Traffic Signal Timing Manual can be used to justify the necessity of a protected left-turn phase. 8-26 Using the results of Problem 8-16, determine the queue service length for each phase assuming the proportion of vehicles arriving during the green interval is 0.8 for the north and south approaches and 0.85 for the east and west approaches. Using Equation 8.19: E-W left: (1 − ) = 3,600 (177 3600⁄ ) × 163(1 − 0.85) = 0.85 − (177 3600⁄ ) × 163 35 = 14.46 E-W thru: (1 − ) = 3,600 (771 3600⁄ ) × 163(1 − 0.85) = 0.85 − (771 3600⁄ ) × 163 49 = 22.99 N-S left: (1 − ) = 3,600 (140 3600⁄ ) × 163(1 − 0.8) = 0.8 − (140 3600⁄ ) × 163 35 = 9.54 N-S thru: (1 − ) = 3,600 (589 3600⁄ ) × 163(1 − 0.8) = 0.8 − (589 3600⁄ ) × 163 38 = 19.62 8-27 Briefly discuss the different methods by which freeway entrance ramps can be controlled. Clearly indicate the advantages and disadvantages of each method, and give the conditions under which each of them can be used. The methods for controlling freeway entrance ramps are closure, simple metering, traffic responsive metering, and integrated system control. Closure entails the physical closure of the ramp by using “Do Not Enter” signs or by placing barriers at the entrance to the ramp. This form of control is the simplest, but also the most restrictive and should be used only when absolutely necessary. Simple metering consists of setting up a pretimed signal with extremely short cycles at the ramp entrance. Simple metering can be used to reduce normal ramp capacity (about 1200 veh/h) to about 250 veh/h, by changing the signal timings of the ramp meter. This type of metering can be used to improve flow on the mainline freeway by determining the difference between the downstream volume and the upstream capacity and setting the metering ramp to match this, or to improve safety at the merge area by allowing only one vehicle at a time to merge. Traffic-responsive metering systems are based on the same principles as the simple metering systems but add the ability to base the metering rate on current traffic conditions rather than predetermined timing plans based on historic data. This type of system therefore has the ability to respond to short-term changes in conditions. Integrated system control brings several ramps together and controls them as a group rather than individually, without concern for how they are impacting one another. This allows the metering rates to be set to maximize the available mainline capacity and improve overall system flow. 8-28 Compare and contrast the different metering systems that are used in traffic signal ramp control indicating under what conditions you will use each. Ramp metering control systems can be divided into two general categories: pre-timed and traffic response. The traffic response category can be further divided into local traffic responsive and systemwide traffic responsive. Pre-timed systems involve the use of traffic data only from a historical perspective; therefore their operations cannot be altered in an automated real-time environment and therefore do not require communication with a traffic management center (TMC). In contrast, traffic response systems utilize current traffic data as an input to their control algorithms. This category can therefore respond to changing traffic conditions if traffic data being collected are made available to the control algorithm. Local traffic responsive control is based only on conditions immediately upstream and downstream of the ramp junction, while systemwide traffic responsive systems allow ramp meter control to be based on a corridor or systemwide optimization of traffic flow. Pre-timed control would typically be used for an isolated location or in an area without traffic monitoring or real-time data collection capabilities. Traffic response control would require real-time data collection; local traffic responsive control would typically be used for isolated locations or locations without communications capabilities, while systemwide traffic responsive control would typically be used in a system with many ramp meters along a corridor and the ability to communicate with a TMC. In the event of a communications loss or other failure of a system wide control algorithm, these systems can be programmed to revert to a local traffic responsive control or to a pre-timed algorithm. Solution Manual for Traffic and Highway Engineering Nicholas J. Garber, Lester A. Hoel 9781133605157
