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111 Chapter Nine CONTROL TYPE, QUEUING, AND LEVEL OF SERVICE ISSUES One common purpose in performing a capacity and level of service analysis of an unsignalized intersection is to determine whether the lane configurations are adequate. If they are and operations are s~1 unsatisfactory, Me analysis can be used to determine whether signalization is needed. In this section, a comparison is made between Me peak hour signal warrant of the Manual of Uniform Trailic Control Devices (MUTCD), and Me recommended capacity and LOS procedure. Then, the properties of average queue on an approach are discussed. Finally, some issues are raised regarding the use of any single measure of effectiveness such as delay, queue, or volume/ca¢Dacibr when detemun~n~ the annroDriate control Ode for an intersection. MllTCD SIGNAL ^/ARRANrS _= _-- War - ~¢ Denomination of an appropriate control for an intersection, either signal control or some form of stop condor, is now made by integrating information from several sources. Traffic signal warrants, level of service analyses, accident data, and public complaints form the basis for a decision to signalize an intersection or to change to stop control. Three doc~ents, among others, are available to assist the traffic engineer In this assessment: the MUTCD, the ITE TraBic Engineering Handbook (TEH) (Pline, 1992), and the HCM. The MUTCD provides a set of warrants to help determine the appropriate conditions for signalization, two-way stop- control, or all-way stop-control. The following 11 signal warrants are provided In the MUTCD: (1) minimum vehicular volume, (2) interruption of continuous traffic, (3) minimum pedestrian volume, (4) school crossings, (5) progressive movement, (6) accident experience, (7) systems, (~) combination of warrants, (9) four hour volumes, (10) peak hour delay, and (l I) peak hour volume. Although only one of these warrants is required to be met before a signal is recommended, traffic engineers should ideally consider all these aspects when making a decision regarding an intersection control type. This set of warrants represents guidance based on collective professional consensus accumulated over many decades. Practicing traffic engineers can refer to these warrants whenever issues arise regarding decisions on intersection control type ;. The MUTCD is c~Tcuttr~dergo~ng a revision due for publication in 1996. Many of Me above warrants are likely to be modified. A draft of the changes includes recommendations to combine some warrants into single, albeit multiple criteria (and/or3 warrants. Even though the existing warrant criteria will be retained, combining warrants should have the effect of malting them more stringent in the aggregate. The TEH points out that traffic signals do not always increase safety and reduce delay. Therefore, it is not appropriate to install signals, regardless of He traffic volume conditions. The TEH provides the following warrants for all-way stop-control: (~) where traffic signals are warranted, multiway stop control is an interim measure Hat can be installed quickly while arrangements are being made for a signal, (2) when an accident problem as indicated by five or more reported accidents in a 12 month penod is of a type Hat can be corrected using a multiway stop, and less restrictive controls have not been successfill, and (3) nununum traffic volumes, (a) where the total vehicle volume entering the intersection Dom all approaches averages at least 500 veh/hr for any 8 hours of an average day, and (b) where the combined vehicular and pedestrian volume from minor stunts averages at least 200 units per hour for the same 8 hours with an average delay to minor street traDic of at least 30 sec/veh during the maximum hour, but when the 85~ch percentile approach speed of the minor street traffic exceeds 40 mph, the minimum volume warrants are 70 percent of the above requirement in (a). Regarding haDic signalwarrants, the TEH states: "Traffic signals Hat are appropriately justified, properly designed, and effectivetr operated can be expected to achieve one or more of He following: (~) to effect orderly traffic movement Trough an appropriate assignment of nght-of- way, (2) to provide for the progressive flow of a platoon of manic along a given route, (3) to interrupt heavy traffic at intervals to allow pedestrians and cross-street tragic to cross or to enter He main street flow, (4) to increase the Aim handing ability of an intersection, or (5) to reduce He Dequency of occurrence of certain types of accidents". Although these two available resources pronde general guidance in determining intersection control type, Hey have an obvious omission: warrants based on a traffic
112 operations perspective. Delay is one of the major measures of effectiveness Hat most traffic engineers have to deal with. There are several factors that affect the vehicle delay at ~ntersechons, such as traffic volume distributions, gap acceptance parameters, and composition of Ming movements. However, none of He warrants has ever covered this issue directly. Because volume data are commonly measmed or easily obtainable, it has become a surrogate for other M.O.E.'s such as delay. The proposed revision of He MUTED warrants implicitly recognizes this lack of operational analysis by requiring that a "traffic engineering study" be used as a basis for dete~ung the need for a change ~ intersection control, as well as recommending Hat delay and gap availability data be collected. This opens He way for an operationally based signal warrant. The 1994 HCM update provides methodologies to compute delay and levy of senice based on certain traffic volume and intersection conditions given certain types of control. With the development of new models and procedures of estimating capacity and delay at stop- controDed intersections, it is now theoretically possible to compare different intersection control types from an operational perspective. Two major tasks of this chapter are: (~) to assess the proposed models and procedures for estimating delay and level of senice at stop-controDed Ions, and (2) to compare the mode} results USA the warrants provided by the MUTCD. The most pertinent MUTED warrants to assess are the peak hour volume (#I ~) and peak hour delay (#10) warrants. These tasks are conducted through use of a sample calculation based on specified traffic volume and intersection geometry conditions. This example then leads to a general discussion of suitable operational M.O.E.'s and warrants. 1h the sample calculations, intersection geometry for both TWSC and AWSC was assumed to consist of 4 legs with a single shared lane on each approach. For signalized Scions, single lane approaches were assumed for ad He approaches; however, a separate left turn bay was assumed forge major street approaches. This assumption was made because on the practical side, a traffic signal would not typically be installed without some geometric improvements being made at least to the major street. Different volume distributions usually exist for intersections with different control types. Table 52 summarizes the volume distributions and turning movements observed from the intersection data collected during this project. For TWSC intersections, Minor ~ is the highest volume subject approach and Major ~ is He major street approach on the leR side of the subject approach For AWSC intersections, Major ~ is always the approach I the highest volume, and Minor ~ is the approach on the right hand side of He Major ~ approach. For He purpose of companson, the volume split on the minor street was based on the conditions usually observed at TWSC Neons. Calculations were conducted based on the total major street volume ranging from 100 veh/hr to 1,800 vower, and the total minor street volume ranging from 100 veh/hr to 1,000 veh/hr. (the above specification of He sample problem already raises a major criticism with current warrant procedures: they only apply to a limited set of unspecified (assumed averaged volume and geometric assumptions that cannot mode} the actual range and complexity of actual traffic operations and geometry).
113 Table 52. Volume and Turning Movements for Sample Calculation ·' ''''it ~= 5'""~3'''''"''""''""'~"~'1"' 22'2''22''''''i''' "'''''"'""'i '''"""''1 [rwsc~ 38%~ 4%~ 84%~ 12%~ 43%~ 18', ~ 78% 1 1 WSC~ 35%~ 18%~ 62%~ 20%~ 30%~ 18' ~ 62% Average37%11%73%16%37%18% 70% t Used btA~b ~1 10%1 7s%1 15% ~1 154 e 1 7o% Approach SplitSO% SO% t~ 2~2~2~-~2~ 2~ 2s~ ~- . ~e - : ~: . ~. ~. ~ : 1 row 1 14% 1 26% 1 17% 1 56% 1 5% 1 36' . 1 31% ' AWSC 15% 18% 62% 20% 20% 18% 62% Average 14.S% 22% 40% 38% 13% 27% 47% UsedinAnsly ~1 120% T40% ~40% I T25., 150% ~1 Approach Split 70% 30% Calculations of delay and level of service for TWSC and AWSC intersections were based on the recommended models and procedures in the NCHRP 3-46 project. For TWSC intersections, Harders basic capacity mode! including impedance and the 1994 HEM delay equation were used The cntical gaps and foHow-up times were all based on the general recommendations of the NCHRP 3- 46. For AWSC ~ntersecdons, Richardson's capacitor mode} and the 1994 HCM delay mode} were used. The saturation --headways were based on five different cases. For signalized intersections, the procedure was based on He methodology In Chapter 9 of the 1994 HCM Update. An Important adjustment to note is Hat He stopped delay output of the signalized intersection methodology was multiplied by the generally recommended I.3 conversion factor to equate it with the total queue delay assumed by the unsignalized intersection delay model. The following parameters were assumed for the signalized intersection calculations: PHF Cycle Length Minimum Green on Minor Minimum Green on Major Lost Time I.0 60 see 5.0sec 10.0 see 9.0 see 4% 2056 12% IS% 33% 20% 2r~ Us" Minor St. Phasing Major St. Phasing Permitted Left Protected + Permitted A set of figures were developed based on different criteria to selectee best control type. Figure 64 shows He average intersection delay per vehicle of each volume combination for He Tree control types. Two peak hour warrant curves from the MUTCD were plotted on the figure for later comparison. Figure 65 illustrates He best intersection control type based on the minimum intersection delay per vehicle. It is encouraging that He result followers He same trend as the MUTCD curves, although signals would be warranted at slightly lower volumes. AWSC control is best applied when a balanced volume distnbution on the major and minor street of intermediate magnitude is achieved Under these circumstances, it can be used as an interim measure before upgrading to signal control. Although the MUTCD warrant lines pronde slightly higher signal warrants Han the result shown here, in practice the installation and maintenance cost of signals would be a consideration in addition to vehicle delay. Signals would be installed only if significant improvements could be achieved in terms of traffic operations. If a significance level of 5 seconds was assumed, i.e., signals would not be install unless the
114 average ~n~section delay per vehicle would be reduced by at least 5 sec/veh, a new figure was developed and is shown as Figure 66. Under this assumption, the preferred control becomes very close to the MUTED warrant curves. T i T T · , i T ~T i ~. ~--- ~j T ~ , ~ . L - SCAM ! ! D _ _2 ~._u6 ~_ ~_. ~_.~. ! i 700 630 sat 560 _490 `~420 Q hi: ~350 o3280- t t -210- ~ ...L.... ~ o ! i ~140 70 o · · · lL~8 ~ ~32 I'm B116 9/11 ors _.._.... ._..l._eaN_..l...._...I..~ ___ I____ .... _ ~-:. ~45~` ~ 18~1 .. 1.~_~. ~_ ~ i. ·---~. ! ! ! ! ! ! ! T T ._. ....~43~5~_... ~...._.... ~ i I. ...._.... !.._....1.. - - l¢,, + ~ - - i~ ~ ~o~_~_~4-~ t~1~1~4~ 0~ . ! ! ! ! ! ! -~--~-; ~ '`; 5 ~ ! i ! ! i i i i I i i i I - -~_~7~40- ~o _iw10 ~u~ ~-U-Ki-1~W1; ~-~14~1~2~4W1~A~Y/UL -:-2~-~-~-~i-u~i _ uL_ 7,:.-~i~ ·---~.~----t~---·~--- - ·~. ·-·~--~4~6- ~-·~-~--~=-44~ i T ~ j ~ 5 15 j j j j j j j j . ~ ~ ~ ~ i ~i ~i i I I 1 1 1 1 1 1 1 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 Total Major St. Vol. (veh/hr) Figure 64. Average Total ~tersection Delay for Different Control Types
115 700 - ~ 630 - . 560 2 490- ~ 420 ¢ 350 a) ·_ Q 280 cn ~ 210 o 1 40 70 o 1 ~ e, ~ ~ ; ; 5-s--s S---5 ~- I - ~--t _~_~_~. g~_~. ~g...~,. i _ 1.._...L _ i L ! ~l ! ! ! ! ! i i . i i _~. _ ~_ % _1_~.~- ~. ^__~__~_~_~_ ~ _ ~ _ ~ ~ Lll-~-'l\~+ i`, ~ ', , ~i'''--~i,~' ~ -'t'''''T---i~ I j j . . ---~ ; .- ~---S--S. S---S. -.$-T--T- t--t-1 1 --t-- --~----~-:--s:'--s`; ~--s--S-- -s--s--s- - ~-r-r-i,---Ti--~-~---S~--S~--s--~---~.---s.--s---s.---1 I r-r-- ~ Ti-I,--T,--~---s--~-~-6 s-~7~--s,--~--~' I _..~-g- T-~ ~T ~ ;~$ ~ t t T T T ~ - T- - T--t-- ,t---t--~-- ---$-i---s,---~ j . , . j . , j , , . .. , i, . i 0 1 00 200 300 400 soo 600 700 800 900 1 000 1 1 00 1 200 1 300 1 400 1 500 1 6m 1 7m 1 ~m Total Major St. Vol. (veh/hr) 1 Figure 65. Op~num Control Type Based on Minimum Average Idtersection Delay
6 7~) 560 , 490 420 a: _. In o .' ~r ` ~I ~! ! ! I ! ! 280 210 1~ t - 70 g g g g g g ·. 6~._ ~_ ~-~-- $-By. 4. i i i ~ i i i i i OTT T T -AT --at;- I 1~-Act- At- _' 5 .... _.... _ . ~ ._...~_ ..,L _.+ ~ _ i _ W~ ~_~-___ ~_~ ~ -__ __ ii _ ii _ An_ T _ ~ _ ~ _ i _ ~ i i ~ it _ _- An-- T.- ~ ---T- - --T.-- - - T--+ _ _ ~_ _ ~. ~._ _ T . _ ~_ F _ ~_ ~_ T . ~_ ~_ _ _ · 9 ~. . . . i . . O- i i i i i i i i , i i i ~I i , , ~, , ~, , ~ ~, ~, I I . , 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 Total Major St. Vol. (veh/hr) Figure 66. Optimum Control Type Based on the Minimum Average Intersection Delay and a 5-Second Significant Difference Level Another set offigures was developed based on intersection level of service. The HEM uses different delay thresholds to designate level of service based on vehicle delay. This is because drivers are believed to have different expectations of reasonable delay at signalized intersections, all-way stop-controlled, and two-way stop- controlled intersections. Figure 67 shows We level of service of each volume combination for Me three control types. Figure 68 was developed teased on Figure 67, which was adapted to show the best intersection control type. Intersecdon control types were shown simultaneously if they rested in the same level of service. Whenever the same level of service was obtained by both Me signalized intersection and stop-controlled intersection, stop condor should be preferred to signal control. Again, a similar trend was observed compared with the MUTCD warrant curves, except that the MUTCD tends to provide stricter (higher) warrants. Similar to Figure 66, a significant diffel-e'ice level could tee applied to Figure 67 to determine Me best ~ntersechon control type.
117 700 -, ! ! . NAlPlC N - lF I - PJr Id 630 ,~ 560 ~490 o > 420 ,'; 350 ·_ 280 cn 210 o 140 70 O L.~ !_1___L..._L.4 I. i . i ~ ~ i, ! ~ _- 1 i ! ! .t 1-'t .\~1 i i I -to-t At-__ ! ! .,,,_ ~--~li,B- ~-'~,~-' .~ I '. 'I i i i_L~i I ~ ~ i ~-it= - .A3- JAN ·- Jam- W4~iA- b`ELfA -A ~ ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! i i ~I i I i I i i i I i i I I i 0 100 200 300 400 500 600 700 800 900 1000 1 100 1200 1300 1400 1500 1600 1700 1800 Total Major St. Vol. (veh/hr) · -1- -1- ~ - 7- 1- 1 I ~ ! \ i i i i i aft 1 ~ _ 4~ F~ ~ ~ .~.~..~ ! ! ! I ! ! I I ~ i i-- i ,~,~.,~,-~,~, ~,~_~1_.~.~ i~ ~ i .~._ - ! i i i i , I, ~ i W%~`, , , , , , , ·-·-7-'-----~----~B&}------~Bl~-----~}'-----~ --·;--M~-~----~.` -N~--~B---NAf`E - -NA~`E - -NA~`B-NdJF`B U~IE-_U FIC FIC Fk] FJB . 'lB ,' `~% ! ~! ! ! i ! Figure 67. Intersechon Level of Servic\e a`OS) for Different Control Types
700 630 ~ ~0 an, 490 ~ 420 Q ~ 350- 280- cn In 210 o 1~. 70 O ·-_ 1 _ ~_ 1 ~1 _ ~_.~. 1., _ _ i . __ ~ ~ _ ___ .. i ;, ~_s ~ ~ _~ Is,_. 1 i_ ~_t ~ -~ 5 ___-,:- .- ! T. IS- -~- -I.. -+_~ - ~ 6_ .... .. j ~ j ~ ~ ~ '-~1 _ j i i i i i ~t ~V~ ~6 at! - -em---a ---a 5-ma__ ~- ~ ---;_$ --S --~- -S --a- _ ' ' ! ,, AS am''' TB ~S,--''''-S-'''a-'''' 1 i 1 1 ~ ~ ~ ~ i ~ i i i ~ ~ ~ ~ i , I I I I I I I , 0 100 700 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 Total Major St. Vol. (veh/hr) Figure 68. Optimum Control Type Based on Intersection LOS The result of the sample calculations show that, for the given assumptions of turning movement proportions, geometry and signal parameters, Here is a fairly good correlation between the MUTED signal peak hour signal warrants and He result obtained based on the operational assessment through~e HCM delay models. However, the HCM probes provide a useful too} for traffic engineers to deal with more specific and diverse traffic and geometric conditions when determining intersection control type. Detenn~n~ng an intersection control type is a complicated process and needs to consider various criteria, many of which are covered by the MUTCD. Therefore, it is not reasonable to expect a single guideline to be able to provide a direct answer to the question of optimum intersection control types. The MUTCD is only able to provide ''planning-level,' guidelines based on very general typical conditions. As such, the MUTCD warrants are no substitute for specific traffic operational studies, which should be conducted before making decisions on intersection control type. QUEUE LENGTH AS A WARRANT Delay appears to be one of the most mean~ngfid M.O.E.'s on which to base signal warrants. However, queue lengths are a visible manifestation of delay and are directly visible. in a comprehensive investigation of existing and possible warrants for traffic signals, Sampson (1992) has shown that queue length would make a suitable operational warrant. He has recommended a Four/S~x Queue Warrant. The warrant is met if any individual queue of vehicles or pedestrians at an intersection exceeds an average of four, or if the sum of ad the queues of vehicles and/or pedestnans exceeds ab average of six, in He peak hour. This warrant's M.O.E. is appealing because it has been showobyGeriouth end Wagner (1967) that mean system delay is strongly correlated wig mean delay in queue and mean queue length. The number of vehicles in a queue is directly proportional to the arrival rate and the amount of time spentin the system. Little(1961),using queuing theory, proved: E(n) = v x E(w) (161) where E(n) is the expected number in system (average
119 queue of vehicles), E(w) is the expected waiting time before leaving the system, i.e., average delay in hours per vehicle), and v is the flow of arrivals (vehicles per hour). Also, it can be shown ~at: E(~) = v X E(w) = E(n) (162) where E(t) Is Me expected total delay (vehicle-hours~our). The expected total delay (vehicle-hours per hour) equals the expected number of vehicles in the average queue. This simple equation proves Mat Me total hourly delay and the average queue are numencaDy identical. Four vehicle- hours/hour of delay can be used interchangeably with an average queue length of four during the hour. The above expressions are particularly valuable because they hold regardless of whether the intersection being studied is isolated, on an arsenal, or in a network; regardless of the type of control, and regardless of whether a single approach or total intersection is considered. Queuing theory, however, underestimates the physical length of queue, since, In practice, vehicles have a finite spacing and vehicles stop at the back of Me queue, which can be some distance upstream of the service point fin this case the stop line). InNCHRP Report 249 (Henry, et al., 1982), it was found actual queue length was related to stopped delay as follows: Stopped delay= 55/60 x queue (163) Henry regarded the main advantages of a queue warrant to be that it can be readily communicated to the public, and it takes He relative ease of the minor right turn into account. In summary, the MUTCD warrants can be shown to be valid under particular circumstances and may be suitable for planrung-leve} analyses. However, they lack the flexibility of an operationally based, case-specific traffic engineering study that may use delay or queue comparisons under different control assumptions as the M.O.E.. It is recommended that further research be conducted to gain a deeper understanding of which operational factors, and their specific values, should be used in practical intersection condom decisions. IMPLICATIONS OF USING AVERAGE DELAY AS THE ONLY CRITERIA FOR LEVEL OF SERVICE The 1994 HCM defines level of service at a TWSC intersection based on the movement that experiences the highest average tote delay. This is unlike the method used for signalized intersections, which is based on a weighted average of He delay per vehicle on all movements. In most cases at TWSC intersections He critical movement is He minor street lefc-turn movement. As such, the minor street left-turn movement typically defines He overall level of service forge intersection. The 1994 HCM sets the lower threshold for level of senice "F" at 45 seconds of delay per vehicle. There are many instances, particularly in urban areas, In which He delay equations used in die both He 1994 HEM procedure and He procedure proposed in this report will predict delays of 45 seconds ([eve] of service "F") or more for minor street movements under very low volume conditions on He minor street (less Han 25 vehicles per hour). The recommended delay mode} for TWSC intersections is given in He following formula. D ~3600 ~ 900T ~ Y _ 1 ~ :t Y _ 1) ~ ~ (164) where D is the average vehicle delay, sec/veh; c is the capacitor, Whir; V is He traffic volume, Whir; and T is He analysis time penod, hr. As can be seen, delay is a function of the degree of saturation, v/c, and capacity is one of the key parameters in the delay model. The first term of the delay equation considers only movement cap acit',r. The level of service "F" threshold of 45 sec/veh is reached with a movement capacity of approximately 85 veh/hr or less. The 1994 HEM capacity procedure assumes random arnvals on the major street. For a typical four-lane arterial wig average daily traffic volumes in the range of 15,000 to 20,000 vehicles per day (peak hour 1,500 to 2,000 veldt), the delay equation used in the TWSC capacity analysis procedure will predict 45 seconds of delay or more ([eve! of senice "F") for most, if not all, TWSC intersections Hat allow minor street lefc-tum movements. The LOS "F" threshold will be reached regardless of the volume of minor street left-turning traffic. Notwithstanding this fact, most low volllme minor street approaches would not meet any of He MUTCD volume or
120 delay warrants for signalization (since the warrants define en asymptote at lOO veh~r once minor approach). As a result, many public agencies that use the HCM level of service thresholds to determine Me design adequacy of TWSC intersections may be forced to eliminate the minor street lefc-turn movement, even when the movement may not present any operation~problem, such as Me formation REGION 2 Queue/Delay Relationship \ ~ - ~ 100 on a_ ce ~ 10 cat of long queues on the minor street or driveway approach. Note that if the effects of 2-stage gap acceptance, flared mirror approaches, an upstream signals were to be incorporated into the procedure, Me capacibr used to calculate delay In Equation 164 would be higher, and the resulting delay lower. REGION 3 1 7YY4 11~;M L{J:~- ''~'' _ . . . . ... , , , l, ,- - v/c - 0.2 v/c - 0.4 v/c - 0.6 V/C- 0.9 v/c- 1.2 0/ 1 10 ~ --100 via- 1 4 Average Queue REGION 1 Figure 69. Illushadon of Queue/Delay Relationship This point is illustrated more clearly in Figure 69, which presents plots of average delay and average queue lengths for an individual movement with volume/capacity ratios varying from 0.2 to I.4. The points on each of the v/c lines are for vogues ranging from a low of 10 vehicles per hour to a high of 700 vehicles per hour. As can be seen from the figure, the current level of service "F" threshold of 45 sec/veh based solely on average delay can be exceeded under many low volume, low v/c, and low queue conditions. Of concern is the region of the graph (denoted as Region 2) that includes v/c ratios less than 1.0 and average delays "mater than 45 seconds. In this region, Me average queue length is typically less than one vehicle, which indicates ~at, although Rivers would likely experience relatively long delays, it is unlikely that long queues would form due to the low demand volumes. There ~ also condidons where average delays wiD be less than 45 seconds per vehicle, but drivers would be faced with very long queues as can be seen in the far right area of Region ~ of the graph. lhis would represent conditions when there is a long queue that is being served relatively fast. Region 3 of tile graph includes volume/capaciD,r ratios greater than l.O. In this region drivers would be faced with ex remely long queues, extremely long delays, or both. Inevalllatirg the performance of TWSC intersections it is important to consider over measures of effectiveness such as v/c ratios for individual movements, average queue
121 lengths, and 95th percentile queue lengths (not shown here, but provided in the 1994 HEM procedure). By focusing on a single MOE for the worst movement only, such as delay for the minor street left turn, users may make inappropriate traffic control decisions. The potential for making inappropriate traffic control decisions is likely to be particularly pronounced when Me HEM level of service thresholds are adopted as a legal standards, as is the case mmany public agencies. ~ recognition of the importance the level of senice designation has assumed in making haiTic control decisions, it may be appropriate to either reconsider the delay thresholds, or reconsider the concept of level of service for the overall intersection based solely on the worst movement, or both. For example, the delay threshold for LOS "F" at signalized intersectionsis60 seconds/vehicle. Signalized~ntersection delay MOE is defined as average stopped delay, whereas msignalized intersection delay is defined as average total delay. A relationship that is commonly used between Me two definitions of delay is that total delay = I.3 x stopped delay. IfLOS"F"forunsignalized intersections were to be set at the same delay threshold as for signalized intersections so that direct comparisons could be made In the context of the control decision, Hen He LOS "F" threshold for Signalized Intersections should be set at approximately 80 sec/veh. Table 53 suggests new LOS thresholds based on delay to the worst movement. This would enable more valid comparisons between signal control and TWSC for a g~venb~ing movement, based on delay. Table 53. Suggested New LOS Threshold 4' ~_~_ s 6.S B 6.6~ l9.S C 19.6 ~32.S D 32.6 ~ S2.0 E S2.1 ~ 78.0 F >80.0 Another improvement to compare control type; would be define the MOE for unsi~alized Intersections to be a weighted average of all delayed movements. While still not accounting for the majority of major street through vehicles, this would make comparisons of the MOE between signals and stop control more valid. There are a number of important implications resulting from the use of average delay as the sole basis for deter ng level of service, particularly when only looldng et the worst movement at the intersection One of the primary motivations for using average delay in He 1994 HEM as opposed to reserve capacity (as was used In He 1985 HCM) was to pronde He user community a more direct and consistent way to compare unsignalized and signalized intersection operations. In doing so however, it is important that users understand there are over operational Indicators that must be considered as well as average delay.
122
