National Academies Press: OpenBook

Design and Management of Historic Roads (2012)

Chapter: Appendix A

« Previous: 5.0 Highway Design; Past, Present and Balanced
Page 100
Suggested Citation:"Appendix A." National Academies of Sciences, Engineering, and Medicine. 2012. Design and Management of Historic Roads. Washington, DC: The National Academies Press. doi: 10.17226/22790.
×
Page 100
Page 101
Suggested Citation:"Appendix A." National Academies of Sciences, Engineering, and Medicine. 2012. Design and Management of Historic Roads. Washington, DC: The National Academies Press. doi: 10.17226/22790.
×
Page 101
Page 102
Suggested Citation:"Appendix A." National Academies of Sciences, Engineering, and Medicine. 2012. Design and Management of Historic Roads. Washington, DC: The National Academies Press. doi: 10.17226/22790.
×
Page 102
Page 103
Suggested Citation:"Appendix A." National Academies of Sciences, Engineering, and Medicine. 2012. Design and Management of Historic Roads. Washington, DC: The National Academies Press. doi: 10.17226/22790.
×
Page 103
Page 104
Suggested Citation:"Appendix A." National Academies of Sciences, Engineering, and Medicine. 2012. Design and Management of Historic Roads. Washington, DC: The National Academies Press. doi: 10.17226/22790.
×
Page 104
Page 105
Suggested Citation:"Appendix A." National Academies of Sciences, Engineering, and Medicine. 2012. Design and Management of Historic Roads. Washington, DC: The National Academies Press. doi: 10.17226/22790.
×
Page 105
Page 106
Suggested Citation:"Appendix A." National Academies of Sciences, Engineering, and Medicine. 2012. Design and Management of Historic Roads. Washington, DC: The National Academies Press. doi: 10.17226/22790.
×
Page 106

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

A-1 APPENDIX A: FACTORS ASSOCIATED WITH INTERSECTION DESIGN AND OPERATIONS 1 Introduction Understanding how movements at intersections are controlled facilitates developing and considering alternatives to minimize the enlargement of intersections while increasing their safety and efficiency. Such alternatives may facilitate efforts to preserve or protect historic features of roadways that otherwise would be removed or destroyed if additional lanes were added to existing intersections. 2 Defining Intersections Roadways consist of a combination of roadway segments and intersections. Roadways are typically defined as “the portion of a highway, including the shoulders, for vehicular use.” For example, a divided highway has two or more roadways. A roadway segment consists of a continuous portion of roadway with similar geometric, operational, and vehicular characteristics. By contrast an intersection is defined as “the general area where two or more roadways join or cross, including the roadway and roadside facilities for traffic movements within the area.” Intersections are complex due to the additional traffic conflicts that occur at them, the need for the assignment of right-of-way for one roadway or movement over another, and the special lane usage that may be required to separate traffic into the desired movements. This makes their design and improvement a challenge, especially in settings with constraints like historic districts or on historic roads. Operation of an intersection is primarily dependent on two variables: (1) an adequate number of lanes to accommodate the traffic volumes for each movement within the intersection; and (2) the amount of time each movement is permitted to move. In order to minimize the need for the addition of lanes to existing intersections, the following considerations for increasing their safety and efficiency should be considered in the sequential order that the treatments are presented. 3 Assignment of Right-of-Way At all intersections the assignment of right-of-way must be provided for one movement at a time over the other conflicting movements. Typically, the assignment of right-of-way is provided by yield signs, stop signs or traffic signals. Information pertaining to the use of these traffic control devices is contained in the Manual of Uniform Traffic Control Devices (MUTCD) that is published by the Federal Highway Administration (FHWA), or in state DOT versions that must comply with MUTCD’s requirements. In most instances, the addition of signage should not be considered an adverse effect on a historic road or a road in a historic setting.

Appendix A: Factors Associated with Intersection Design & Operations A-2 3.1 Assignment by Signing Where traffic volumes are extremely low on both the major (road with higher traffic volume) and the minor (road with lower traffic volume) crossing roads, yield signs may be erected on the minor roadway to assign right-of-way to the major roadway and to inform users on the minor roadway that they must yield. As the traffic volumes increase on the major roadway and gaps in the traffic flow between vehicles are reduced, the next step in the assignment of right-of-way is to erect stop signs on the minor roadway for safety purposes. Requiring vehicles to stop generally provides more time for them to analyze the traffic flow on the major roadway and be more assured in finding safe gaps in the major roadway’s traffic stream for either entering the roadway or crossing it. Where traffic volumes increase on both the major and minor crossroads and it is relatively equal and does not meet the traffic warrants for a signal, stop signs may be erected on both approaches of both roadways (four-way stop). Four-way stops are used primarily to increase safety, and they help to create additional usable gaps in the traffic stream of both the major and minor crossroad and also provide for the assignment of right-of-way for both roadways. 3.2 Assignment by Traffic Signals Warrants (or criteria) are established in the Manual on Uniform Traffic Control Devices (MUTCD) and must be met before traffic signals can be erected. The purpose of warrants is to provide a guarantee to the public that the need for a signal, which represents an investment to install and operate, is valid based on traffic operations, not for political reasons, as a method to ticket or levy fines, or enhance the movements of one entity over the greater movements of the public. There are a number of different warrants and each base their criteria on an individual item. The items include traffic volumes, pedestrian volumes, number of crashes, signal progression (ability to move from one signal to another without stopping), school crossings, interruption of continuous traffic, peak hour delay, peak hour warrant, combinations of warrants, etc. The addition of traffic signals is not generally considered an adverse effect on a historic road because it is reversible. There are numerous methods for mounting traffic lights and designs and finishes for light standards. Compatibility with historic settings should be a consideration. Where traffic volumes at intersections increase, eventually they may meet the warrant for installing a traffic signal. But meeting the warrant does mandate that it be installed; it simply means it is permissible to install a traffic signal. Meeting the threshold represents the tradeoff point where the delay to motorists from using stop signs equals the delay encounter if the traffic signal were operating. However, as traffic volumes increases on either road, not installing the traffic signal increases delay. When traffic signals are installed, three new elements are added in the assignment of right-of-way; signal phase, signal cycle, and signal-cycle length.

Appendix A: Factors Associated with Intersection Design & Operations A-3 A signal phase is the permitted movements that receive a specified amount of green time plus yellow time and all red time allocated to the movements. The amount of green time displayed to each phase is generally proportional to the highest traffic volume per lane for a movement which is moving during that phase. The signal cycle length is the summation of the green time, yellow time and all red time for each signal phase available within the signal cycle. As an example, a four-phase signal could display green time to; (1) the through and right turning traffic on the minor crossroad; (2) a separate amount of green time in another phase to left-turning traffic on the minor crossroad; (3) a separate amount of green time in another phase to the through and right-turning traffic on the major crossroad; and (4) a separate amount of green time in another phase to the left-turning traffic on the major crossroad. The cycle length for the four-phase signal would equal the summation of the four green times provided plus their related clearance times (yellow light) and all red clearance times before the next phases were started. A traffic signal cycle consists of all the signal phases that occur before they are repeated. In general, the timings for each signal phase should be just long enough to accommodate the traffic volume for those movements within each signal phase. This will minimize delay and the length of the signal cycle, thereby permitting the return of those phases quickly to accommodate traffic that was stopped at the signal since the last signal phase for those movements. When the traffic volumes reach the available capacity of the existing lanes, additional volumes may be moved through the intersection if the cycle length is lengthened. This reduces the number of cycles per hour that in turn reduces the number of times the clearance and all red intervals must be shown when traffic should stop moving, thereby permitting more green time to actually move traffic. As a rule of thumb, the capacity of a lane can be increased to carry more vehicles per hour if a method can be found to reduce the number of signal phases while providing adequate green time to all movements. 4 Lane Additions Installation of a two-phase traffic signal to replace stop signs may be adequate for the major and minor cross roads, especially where both roads are two-lane roadways. As traffic increases, especially on the major cross road, gaps in the opposing traffic will be reduced for left-turning traffic. At some point if traffic continues to increase, it will be necessary to add left-turning lanes on the major crossroad or develop a non-traditional way to maintain an acceptable level of service for the intersection. Lane additions at the intersections of historic roads should be evaluated on a case-by-case basis to determine the effect they may have on the specific features of the road that contribute to its significance. In some instances, lane additions may be accomplished without having any adverse effects on significant features. The determination of when a traffic lane should be added is usually based on a capacity analysis, an accident analysis, or both. Capacity analyses are used to determine the level

Appendix A: Factors Associated with Intersection Design & Operations A-4 of service (LOS) (a graded system of expressing relative amounts of delay incurred by motorists using the facility) and the volume-to- capacity ratio (V/C). The chart below summarizes the graded LOS, from A to F, and provides the range of delay in seconds for each level. Level of Service Criteria – Signalized Intersections Signalized Intersection LOS Control Delay per Vehicle (seconds/vehicles) A 10 B > 10 and 20 C > 20 and 35 D > 35 and 55 E > 55 and 80 F > 80 For interrupted flow conditions, such as at signalized intersections, LOS is defined in terms of total delay, which is a measure of driver discomfort, frustration, fuel consumption, increased travel time due to geometrics, traffic, and incidents. Total delay is the difference between the travel time actually experienced and the travel time that would result in the absence of any delay from traffic control, other vehicles and incidents. LOS A indicates that delay is minimal, progression is extremely favorable, and most vehicles arrive during the green light phase. For signalized intersections, LOS F generally indicates poor progression, long cycle lengths, individual cycle failures and high delays. In addition to the LOS at intersections, the V/C ratio must also be considered. A V/C ratio of 1.00 represents full capacity of the lanes for a specified movement. Typically, engineers and planners would like to design with V/C ratios somewhere around 0.85. This would mean that after the design year is reached for an intersection, approximately 15% of the capacity for that movement would still exist before it was overcapacity. The capacity of a signalized intersection involves a balance between the number of lanes available for any movement and the amount of “green time” from the signal which can be provided to that movement. Typically, the more green time that can be provided results in the least number of lanes required to move the traffic volume for that movement. However, there are only 3600 seconds in an hour, and green time must be divided and assigned to each movement. Using the methodology within the HCM, the number of lanes for any movement for any given LOS can be determined. The methodology also considers the time needed for pedestrians crossing and considers the additional time needed to move trucks, buses and recreational vehicles. The purpose of the left-turn lanes is primarily to move left-turning vehicles out of the through or general purpose lanes so as not to block through and right-turning traffic from

Appendix A: Factors Associated with Intersection Design & Operations A-5 moving through the intersection. Adding left-turn lanes does not necessarily require a separate signal phase for the left turners on the major roadway. But as traffic continues to increase, at some point a separate signal phase for left turners may well become necessary and with it the need to modify the old geometry and provide the needed or required additional lanes. Many state DOTs have developed warrants to determine when it is beneficial based on delay to provide either a left- or right-turning lane. The turn-lane warrants may be based on either/or a combination of traffic volumes for the turning and opposing traffic and the number of crashes due to rear-end conflicts with through traffic. As traffic volumes increase there may be a need for additional through lanes as well. However, before any additional lanes are considered, signal timings and phases should be reviewed to determine if the green time provided to the various phases is being used efficiently, or whether a redistribution of green time to the various phases may permit additional traffic to travel through the intersection without additional lanes. 4.1 Types of Lane Additions When the capacity of an intersection is reached or motorist delays have become substantial, there may be a choice in the type of additional lanes to construct, i.e. through- , left-turning or right-turning lanes. The types of lanes constructed can have a major effect on the historic footprint of the intersection and its roadway segments. Typically, when through lanes are added, they must be added for relatively long distances. This will result in widening not just the intersection but also the roadway far beyond the limits of the intersection. In general, right- or left-turning lanes only widen the roadway on the approaches to the intersection, and for only the length required to permit sufficient stacking or storage within the lane before receiving the green light during their phase. Since the capacity of a movement (i..e. left turn movement) at an intersection is based on the number of lanes available for moving traffic and the amount of green time provided to that movement, it is often possible, for example, to move more through traffic by constructing either an additional right- or left-turning lane. This may have a pronounced effect on minimizing the footprint of the overall width of the corridor. When site conditions permit, it is sometimes possible to add additional turning lanes in order to avoid adding additional through lanes. Since the same turning volumes will be accommodated for the turning movement, the amount of green time for the turning movement can be reduced and then transferred to the through movement. A similar example would be related to water hoses. In general, just as much water could be squirted out a four inch hose in ten minutes as could be squirted out a two inch hose in twenty minutes. While the lanes for the through movements remain the same, the amount of green time to that movement increases.

Appendix A: Factors Associated with Intersection Design & Operations A-6 4.2 Alternatives to Lane Additions Instead of adding lanes, it may be possible to increase intersection capacity by using new intersection designs known collectively as non-traditional intersections. Non-traditional intersections include continuous flow intersections, paraflow intersections, quadrant intersections, roundabouts, and superstreets to mention a few. In general, these intersections provide alternatives to efficiently move left turning vehicles, and thereby decrease the number of signal phases. Non-traditional intersections can increase capacity over their traditional counterparts by 50% – 90%, thereby possibly eliminating the need to add additional lanes at intersections and negate an increase in the width of the footprint. One example of a non-traditional intersection is the “Michigan U-Turns”. An intersection using Michigan U-Turns would restrict all actual left turns at an intersection. However, in order to provide an equivalent movement to the left turn, drivers would continue through the intersection for approximately 600 feet to point where they can make a U-turn. Once they make the U-turn (which could be signalized), they travel back to the intersection and make a right turn, which is equivalent of making a left turn initially. By not allowing left turns at the intersection, the signal phasing can be reduced to 2 phases. As a result, delays and travel times are reduced over what they would have been if left turns were allowed. Typically, the U-turns can be made on the existing pavement. Where additional pavement is needed, small bulbouts, known as loons, can be built without much interference to the existing roadside. Since several of the non-traditional intersections accommodate improvements within existing or minimally modified street patterns, non-traditional intersections can offer flexibility for historic roads and settings. 5 Geometrics The primary geometric elements for intersections are roadway lane width and shoulder width. However, there are other elements that may be considered, such as urban tree lawn width, sidewalk width, median width and islands. Islands may be added to channel vehicles or to provide pedestrian refuges. Many of these elements have a range of values depending on the roadway’s functional classification. When the goal is to minimize the width in order to preserve or maintain historic features, it may be possible to select values from the bottom of the range. Other geometric considerations at intersections include the angle of the intersection – the angle between the two crossing roadways. Preferably, intersection should cross each other at 90˚, but angles of 70˚ are permitted. An angle of 60˚ may be used when the intersection is signalized and the intersection is skewed such that a driver stopped on the side road has the acute angle (at center of intersection) on his left side (vision not blocked by his own vehicle). A final element to consider is the corner radius return that provides the pavement connection for vehicles turning right from one roadway to another. The radius used to connect the two roadways must consider the context of the application. Variables such as whether the intersection is urban or suburban, signalized or unsignalized and the potential conflict with pedestrians must be considered for the safety and convenience of both motorists and pedestrians. In general, the smallest radius possible for the circumstances should be used, rather than one which would accommodate the largest possible design

Appendix A: Factors Associated with Intersection Design & Operations A-7 vehicle, which accounts for less than 2% of total users. Large radii can encourage the speed of turning motorists, which can affect the safety of pedestrians crossing in the crosswalks and lengthen the distance of their crossing; thereby, providing additional exposure to pedestrians.

Design and Management of Historic Roads Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

TRB’s National Cooperative Highway Research Program (NCHRP) Web-Only Document 189: Design and Management of Historic Roads explores how the inherent flexibility in the current policies, manuals, criteria, rules, standards, and data sets that underlie the transportation planning and project development process may be used to preserve historic roads and roads in historic districts and settings.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!