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SICap software from TechSOFT Engineering Services is for Signalized Intersection Capacity analysis in the application of Transportation Engineering and Planning.

Best Features

Introduction of SICap

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SICap may be used for the Analysis of Traffic Saturation in any Urban Street with US traffic system. This has followed the guidelines of “Highway Capacity Manual-HCM 2000” by Transportation Research Board of Washington, USA, to determine the Level of Service (LOS) of a Road Intersection.

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SICap is applicable on Planning Analysis and Operational Analysis. It generates Reports on Saturation Flow, Queue & Result with Single lane and Multi lane Group approach. SICap informs about the Status of the Intersection in terms of Capacity, Level of Service for a given Cycle length and Green time of the Signal.

In Planning analysis

Planning Analysis is used to estimate the green times before performing the more detailed operational analysis. The Quick Estimate method also evaluates delays and level of service based on the estimated signal timing and default values; but to perform a detailed analysis, the user should export the data to the Operational module.

In Operational analysis

The purpose of traffic analysis is the determination of capacity and level of services of lane groups or of the intersection as a whole. Detailed data of geometry, traffic conditions and signalization of the intersection are required. These should be known regarding an existing or a planned intersection. Because the traffic analysis of signalized intersections is very complex, it is worth to divide it into following units:

Input Unit: all data necessary to the calculation are given here. This includes data regarding geometry, traffic volumes, traffic conditions, and signalization.

Volume Adjustment unit: the traffic volume is generally determined for the peak hour in vph. This unit converts hourly traffic volumes to 15 minutes flow rates, and takes into account the effect of the lane configuration. Determination of lane groups is made in this unit as well.

Saturation flow unit: the saturation flow is calculated for each lane group. The ideal saturation flow is adjusted for the specific conditions of the intersection.

Capacity analysis unit: it is based on the flow rates. The saturated flow rates of the capacity and the volume, capacity ratios (v/c) are calculated for the lane groups. The critical lane group is also calculated for the lane groups. The critical (v/c) ratio is also calculated for the intersection as a whole.

Level of service unit: The delays are estimated regarding each lane groups. The average delays of directions and the intersections as a whole are also calculated. The level of services are determined based on the delays.

Input Parameters

Exhibit 16-3 provides a summary of the input information required to conduct an operational analysis for signalized intersections. This information forms the basis for selecting computational values and procedures in the modules that follow. The data needed are detailed and varied and fall into three main categories : geometric, traffic, and signalization.

EXHIBIT 16-3 INPUT DATA NEEDS FOR EACH ANALYSIS LANE GROUP

Type of Condition Parameter
Geometric conditions Area type
Number of lanes, N
Average lane width, W (m)
Grade, G(%)
Existence of exclusive LT or RT Lanes
Length of storage bay, LT or RT Lane, Ls (m)
Parking
Traffic conditions Demand volume by movement, V(veh/h)
Base Saturation flow rate, so (pc/h/In)
Peak hour factor, , PHF
Percent heavy vehicles, (HV (%)
Approach pedestrian flow rate vpsd (p/h)
Local buses stopping at intersection, NB (buses/h)
Parking activity, Nm (maneuvers/h)
Arrival type, AT
Proportion of vehicles arriving on green, P
Approach speed, SA (km/h)
Signalization conditions Cycle length C(s)
Green time, G (s)
Yellow-plus-all-red change–and–clearance interval
(intergreen), Y (s)
Actuated or pretimed operation
Pedestrian push-button
Minimum pedestrian green GP (s)
Phase Plan
Analysis period T (h)

Geometric Conditions

Intersection geometry is generally presented in diagrammatic form and must include all of the relevant information, including approach grades, the number and width of lanes, and parking conditions. The existence of exclusive left-or right-turn lanes should be noted, along with the storage lengths of such lanes.

When the specifics of geometry are to be designed, these features must be assumed for the analysis to continue. State or local policies and guidelines should be used in establishing the trial design. When these are not readily available, Chapter 10 contains suggestions for geometric design that may be useful in preparing an assumed preliminary design for analysis.

Traffic Conditions

Traffic volumes (for oversaturated conditions, demand must be used) for the intersection must be specified for each movement on each approach. These volumes are the flow rates in vehicles per hour for the 15 min analysis period, which is the duration of the typical analysis period ( T=0.25). If the 15-min data are not known, they may be estimated using hourly volumes and peak hour factors (PHFs). In situations where the v/c is greater than about 0.9, control delay is significantly affected by the length of the analysis period. In these cases, if the 15-min flow rate remains relatively constant for more than 15 min, the length of time the flow is constant should be used as the analysis period, T in hours:

If v/c exceeds 1.0 during the analysis period, the length of the analysis period should be extended to cover the period of over-saturation in the same fashion, as long as the average flow during that period is relatively constant. If the resulting analysis period is longer than 15 min and different flow rates can be identified during equal – length sub-periods within the longer analysis period, a multiple period analysis using the procedures in Appendix F should be performed using each of these sub periods individually. The length of the sub periods would normally be, but not be limited to, 15 min. each.

Vehicle type distribution is quantified as the percent of heavy vehicles (% HV) in each movement, where heavy vehicles are defined as those with more than four tires touching the pavement. The number of local buses on each approach should also be identified, including only those buses making stops to pick up or discharge passengers at the intersection (on either the approach or departure side). Buses not making such stops are considered to be heavy vehicles.

An important traffic characteristic that must be quantified to complete an operational analysis of a signalized intersection is the quality of the progression. The parameter that describes this characteristic is the arrival type, AT, for each lane group. Six arrival types for the dominant arrival flow are defined in Exhibit 16-4.

EXHIBIT 16-4 ARRIVAL TYPES

EXHIBIT 16-4 ARRIVAL TYPES

Arrival Type Description
1. Dense platoon containing over 80 percent of the lane group volume, arriving at the start of the red phase. This AT is representative of network links that may experience very poor progression quality as a result of conditions such as overall network signal optimization.
2. Moderately dense platoon arriving in the middle of the red phase or dispersed platoon containing 40 to 80 percent of the lane group volume, arriving throughout the red phase. This AT is representative of unfavorable progression on two-way streets.
3. Random arrivals in which the main platoon contains less than 40 percent of the lane group volume. This AT is representative of operations at isolated and non-interconnected signalized intersections characterized by highly dispersed platoons. It may also be used to represent coordinated operation in which the benefits of progression are minimal.
4. Moderately dense platoon arriving in the middle of the green phase or dispersed platoon containing 40 to 80 percent of the lane group volume, arriving throughout the green phase. This AT is representative of favorable progression on a two way street.
5. Dense to moderately dense platoons containing over 80 percent of the lane group volume, arriving at the start of the green phase. This AT is representative of highly favorable progression quality, which may occur on routes with low to moderate side – street entries and which receive high priority treatment in the signal timing plan.
6. This arrival type is reserved for exceptional progression quality on routes with near ideal progression characteristics. It is representative of very dense platoons progressing over a number of closely spaced intersections with minimal or negligible side-street entries.

The arrival type is best observed in the field but can be approximated by examining time space diagrams for the street in question. The arrival type should be determined as accurately as possible because it will have a significant impact on delay estimates and LOS determination. Although there are no definitive parameters to precisely quantify arrival type, the platoon ratio is computed by Equation 16-1.

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P may be estimated or observed in the field, whereas gi and C are computed from the signal timing. The value of P may not exceed 1.0.

Signalization Conditions

Complete information regarding signalization is needed to perform an analysis. This information includes a phase diagram illustrating the phase plan, cycle length, green times, and change and clearance intervals. Lane groups operating under actuated control must be identified, including the existence of push button pedestrian actuated phases.

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It is assumed that the 15th percentile walking speed of pedestrians crossing a street is 1.2m/s in this computation. This value is intended to accommodate crossing pedestrians who walk at speeds slower than the average. Where local policy uses different criteria for estimating minimum pedestrian crossing requirements, these criteria should be used in lieu of Equation 16-2.

When signal phases are actuated, the cycle length and green times will vary from cycle to cycle in response to demand. To establish values for analysis, the operation of the signal should be observed in the field during the same period that volumes are observed. Average field measured values of cycle length and green time may then be used.

When signal timing is to be established for analysis, state or local policies and procedures should be applied where appropriate. Appendix B contains suggestions for the design of trial signal timing. These suggestions should not be construed to be standards or criteria for signal design. A trial signal timing can not be designed until the volume adjustment and saturation flow rate modules have been completed. In some cases, the computations will be iterative because left turn adjustments for permitted turns used in the saturation flow rate module depend on signal timing. Appendix B also contains suggestions for estimating the timing of an actuated signal if field observations are unavailable.

An operational analysis requires the specification of a signal timing plan for the intersection under study. The planning level application presented in Chapter 10 offers a procedure for establishing a reasonable and effective signal timing plan. This procedure is recommended only for the estimation of LOS and not for the design of an implementable signal timing plan. The signal timing design process is more complicated and involves, for example, iterative checks for minimum green time violations. When phases are traffic actuated, the timing plan will differ for each cycle. The traffic actuated procedure presented in Appendix B can be used to estimate the average cycle length and phase times under these conditions provided that the signal controller settings are available.

The design of an implementable timing plan is a complex and iterative process that can be carried out with the assistance of computer software. Although the methodology presented here is oriented toward the estimation of delay at traffic signals, it was suggested earlier that the computations can be applied iteratively to develop a signal timing plan. Some of the available signal timing software products employ the methodology of this chapter, at least in part.

There are, however, several aspects of signal timing design that are beyond the scope of this manual. One such aspect is the choice of the timing strategy itself. At intersections with traffic actuated phases, the signal timing plan is determined on each cycle by the instantaneous traffic demand and the controller settings. When all of the phases are pretimed, a timing plan design must be developed. Timing plan design and estimation are covered in detail in Appendix B.

LANE GROUPING

The methodology for signalized intersections is disaggregate; that is, it is designed to consider individual intersection approaches and individual lane groups within approaches. Segmenting the intersection into lane groups is a relatively simple process that considers both the geometry of the intersection and the distribution of traffic movements. In general, the smallest number of lane groups is used that adequately describes the operation of the intersection. The following guidelines may be applied.

  • An exclusive left turn lane or lanes should normally be designated as a separate lane group unless there is also a shared left through lane present, in which case the proper lane grouping will depend on the distribution of traffic volume between the movements. The same is true of an exclusive right turn lane.
  • On approaches with exclusive left turn or right turn lanes, or both, all other lanes on the approach would generally be included in a single lane group.
  • When an approach with more than one lane includes a lane that may be used by both left turning vehicles and through vehicles, it is necessary to determine whether equilibrium conditions exist or whether there are so many left turns that the lane essentially acts as an exclusive left turn lane, which is referred to as a de facto left turn lane.

De facto left turn lanes can not be identified effectively untill the proportion of left turns in the shared lane has been computed. If the computed proportion of left turns in the shared lane equals 1.0 (i.e. 100 percent), the shared lane must be considered a de facto left –turn lane.

When two or more lanes are included in a lane group for analysis purposes, all subsequent computations treat these lanes as a single entity. Exhibit 16-5 shows some common lane groups used for analysis.

PHASE PLANS

The most critical aspects of any design of signal timing is the selection of an appropriate phase plan. The phase plan comprises the number of phases to be used and the sequence in which they are implemented. As a general guideline, simple two phase control should be used unless conditions dictate the need for additional phases. Because the change interval between phases contributes to lost time in the cycle, as the number of phases increases, the percentage of the cycle made up of lost time generally also increases.

Exhibit B-16-1 shows a number of common phase plans that may be used with either pretimed or actuated controllers. Exhibit B16-2 illustrates an optional phasing scheme that typically can be implemented only with actuated controllers.

Two Phase Control

Two phase control is the simplest of the available phase plans. Each of two intersecting streets is given a green phase during which all movements on the street are allowed to proceed. All left and right turns are made on a permitted basis against an opposing vehicle flow, pedestrian flow, or both. The two phase plan is shown in Exhibit B16-1 (a). This phase plan is generally used unless turn volumes require protected phasing.

Multiphase Control

Multiphase control is adopted at any intersection where one or more left or right turns require protected phasing. It is generally the left turn movement that requires a partially or fully protected phase. Local policy and practice are critical determinants of this need. Most agencies have guidelines for when left turns require protected phasing. Protected left turn phasing is also considered when the speed of opposing traffic is greater than 65km/h.

Multiphase control can be provided in a variety of ways, depending on the number of turns requiring protected phasing and the sequence and overlaps used. Exhibit B16-1 presents three common plans for multiphase control. Exhibit B16-1(b) shows a three phase plan in which an exclusive left turn phase is provided for both left turn movements on the major street. It is followed by a through phase for both directions of the major street, during which left turns in both directions may be permitted on an optional basis.

The use of a permitted left turn phase following protected left turn phases is very much a matter of local practice. The phasing illustrated in Exhibit B16-1(b) can be used either for protected or protected –permitted operation in either mode. Note that a few agencies use permitted plus protected phasing. Exclusive left turn phases provide for simultaneous movement of opposing left turns and are most efficient when the opposing left turn volumes are nearly equal. When volumes are unequal, or in cases, in which only one left turn requires protected phasing, other phase plans are more efficient.

The three phase plan may be expanded to a four phase sequence if both streets require left turn phases. Such a sequence is shown in Exhibit B16-1(d). Left turns may be continued on a permitted basis concurrent with the through phases. It is common practice to provide exclusive lanes for left or right turns with protected phases.

Exhibit B16-1(c) shows what is commonly referred to as leading and lagging green phasing. The initial phase is a through plus left turn phase for one direction of the major street, followed by a through phase for both directions of the major street, during which left turns in both directions may be permitted on an optional basis. Note that many operating agencies do not, as a matter of policy, use the optional permitted left turn with this type of phasing because of safety considerations. The direction of flow started in the first phase is then stopped, providing the opposing direction with a through plus left turn phase. The final phase accommodates all movements on the minor street.

Such phasing is extremely flexible. When only one left turn requires a protected phase, a leading green can be provided without a lagging green phase. When left turn volumes are unequal, the lengths of the leading and lagging green can be adjusted to avoid excessive green time for one or both left turn movements. Leading or lagging green phase, or both, can even be used where no left turn exists as long as turns are permitted to continue during the through phase. The phasing of Exhibit B-16-1(c) may also be expanded to incorporate leading or lagging green phases on both streets.

All of the phase plans discussed to this point can be implemented with pretimed or actuated controllers. The only difference in operation would be the manner in which green time is allocated to the various phases. For pretimed controllers, green times are preset, whereas for actuated controllers, green times vary on the basis of detector actuations.

At this point, it is necessary to recognize the differences in the way that modern traffic actuated controllers actually implement the phase plan. Exhibit B16-1 depicts a single ring, sequential representation of the phase plan, in which a signal phase is used to indicate the combination of all movements that the proceeding at a given point in time. Modern traffic actuated controllers do not use this scheme. Instead, they implement a dual ring concurrent phasing in which each phase controls only one movement, but two phases are generally being displayed concurrently.

The dual ring concurrent concept is illustrated in Exhibit B16-2. Note that eight phases are shown, each of which accommodates one of the through or left turning movements. A barrier separates the north- south phases from the east-west phases. Any phase in the top group (Ring 1) may be displayed with any phase in the bottom group (Ring 2) on the same side of the barrier with out introducing traffic conflicts. For simplicity, the right turns are omitted and assumed to proceed with the through movements.

The definition of a phase as presented in Exhibit B16-2 is not consistent with that in Exhibit B16-1 nor with the definition given in Chapter 10. It is however, a definition that is universally applied in the traffic control industry. It is the responsibility of the analyst to recognize which definition is applicable to any given situation. For purposes of the capacity and delay analysis procedures presented in the body of this chapter, each lane group is considered to be controlled separately by a phase with specified red, green, yellow, and all red times, so either definition could apply. The examples shown throughout the chapter are based on the single ring sequential concept. However, the dual ring definition must be used for estimating the timing plan at traffic actuated intersections using the procedure presented in this appendix.

The dual ring phases that accommodate left turns will only be used if the left turns are protected. Left turns with compound protection will proceed with their concurrent through movements. For example, none of the left turn phases would be used by a dual ring controller to implement the two phase plan shown in Exhibit B16-1(a). All of the other phase plan examples shown in Exhibit B16-1 may be created by selectively omitting left turn phases and by reversing the order in which the through and left turn phases are displayed in either ring.

The advantage of the dual ring concept is that it is able to generate the optimal phase plan for each cycle in response to the traffic demand. Pretimed controllers, and earlier versions of traffic actuated controllers, are more constrained in this regard. The maximum flexibility is provided by allowing the first (usually left turn) phases in Rings 1 and 2 to terminate independently after their respective demands have been satisfied.

It is also possible to constrain these phases to terminate simultaneously to emulate the older, less efficient equipment. For example, simultaneous termination of the northbound and southbound left turn phases in Exhibit B16-2 would produce the phasing example shown in Exhibit B16-1(b). Independent termination of the two left turns would introduce an overlap phase between the left turn phase and the through movement phases in Exhibit B16-1(b). The overlap phase would be accommodate the heavier volume of the two left turns together with the concurrent through movement, thereby making more effective use of the cycle length. The degree of benefit obtained from phase overlaps of this nature depends on the degree of difference in the opposing left turn volumes.

The establishment of a phase plan is the most creative part of signal design and deserves the careful attention of the analyst. A good phase plan can achieve efficiency in the use of available space and time, whereas an inappropriate plan can cause inefficiency. The phase plans presented and discussed in this appendix represent a sampling of the more common forms used. They may be combined in a number of innovative ways on various approaches of an intersection.

Again, local practice is an important determinant in the selection of a phase plan. Phasing throughout an area should be relatively uniform. The introduction of the protected plus permitted phasing at one location in an area where left turns are generally handled in exclusive left turn phases, for example, may confuse drivers. Thus, system considerations should also be evaluated when phase plans are established.

Allocation of Green Time

The allocation of green time is an important input to the methodology presented earlier in this chapter for the estimation of delay. The average cycle length and effective green time for each lane group must be defined. The most desirable way to obtain these values is by field measurement, however, there are many cases in which field measurement is not possible. For example, the comparison of hypothetical alternatives precludes field measurements. Even for the evaluation of existing conditions, the required data collection is beyond the resources of many agencies.

A procedure for estimating the signal timing characteristics is therefore an important traffic analysis tool. Such a procedure is also useful in designing timing plans that will optimize some aspects of the signal operation. In this respect, pretimed and actuated control must be treated differently because the design and analysis objectives are different. For pretimed control, the objective is to design an implementable timing plan as an end product. In traffic actuated control, the timing plan is generated by the controller itself on the basis of operating parameters that are established for each phase. This operation creates two separate objectives of traffic actuated control. The first is to determine how the controller will respond to a specified combination of operating parameters and traffic conditions. The second is to provide some indication of the optimal values for the key operating parameters.

Time Plan Design for Pretimed Control

The design of an implementable timing plan is a complex and iterative process that is generally carried out with the assistance of computer software. Several software products are available for this purpose, some of which employ, at least in part, the methodology of this Chapter.

Design Strategies

There are several aspects of signal timing design that are beyond the scope of this manual. One such aspect is the choice of the timing strategy itself. Three basic strategies are commonly used for pretimed signals.

Equalizing the v/c ratios for critical lane groups is the simplest strategy and the only one that may be calculated without excessive iteration. It will be described briefly in this appendix. It is also employed in the timing plan synthesis procedures of the planning procedure presented in Chapter 10. Under this strategy, the green time is allocated among the various signal phases in proportion to the flow ratio of the critical lane group for each phase.


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