Capacity and Level of Service at Unsignalized Intersections Final Report Volume 1 - Two-Way-Stop-Controlled Intersections (1996) / Chapter Skim
Currently Skimming:
Chapter Eight. Special Capacity and Delay Issues
Chapter Eight. Special Capacity and Delay Issues
Pages 83-110
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 83... ...
When a TWLTE exists on tiLe major street, the minor street left turn vehicle usually merges into the TWLTE first, Men seeks a usable gap on the other approach while slowly moving for some distance along Me TWLTL. Empirical Data The influence of a TWOS and liaised median on the minor street capacity at TWSC intersections has been investigated using the recommended cap acid mode} and procedure.
|
From page 84... ...
is presented In this section. At many unsignalized intersections there is a space In the center of the major street available where several minor street vehicles can be stored between the traffic flows of the two directions of the major street, especially in the case of multi-lane major traffic.
|
From page 85... ...
If, however, a sufficient gap is provided In the major streams of bow parts of the intersection simultaneously, a minor street vehicle from movement ~ can pass the whole intersection without teeing queued somewhere in the storage area. This is the same situation as exists with single stage gap acceptance.
|
From page 86... ...
Based on these assumptions, the computational steps which are necessary to estimate the capacity of an unsignalized intersection where the minor movements have to cross the major street in two stages are presented In Table 46. Alternatively, graphical methods are presented In the following sections.
|
From page 87... ...
vl= volume of priory sheet leRt~uning traffic et pert I l v2= volume of major street through1raffic comings the left at part I L v'= volume ofthe sum of all major sheet flows coming In the right at part IL | Here the volumes of all prior movements et pert II have to be included. These are: major right (6, except if this movement is guided along a biangular island Ted from We ~roughbaffic)
|
From page 88... ...
maybe computed from other theories than gap acceptance, or they could be measured in the field. gap acceptance theory, the critical gaps tc are different for each part of the ~ntersechom The only necessary condition for the application of these graphs is that the follow-tip times If are of nearly identical magnitude.
|
From page 89... ...
= 342 (veh/hr) (14~ If gap acceptance theory is applied (Equation 135)
|
From page 90... ...
Based on these denvations, a set of graphs was evaluated which enable a simple estimation of the capacity at an unsignalized intersection under two-stage priority. These graphs are ready to be used In practice, nevertheless, an emp~ncal confirmation of this model approach is desirable in filture research Meanwhile, since tills is a commonly encountered phenomenon at TWSC intersections with multilane highways, He theory presented here is recommended for use at unsignalized intersections.
|
From page 91... ...
The depart flow rate was recorded during each 15-minute interval when a continuous queue existed on the minor sweet approach This observed departure flow rate is regarded as the true capacity of He minor street. The minor street approach capacity was also estimated by dinding 3600 by the sum of the service time and move-up time (see chapter four)
|
From page 92... ...
The usual geometric design of an Signalized ~ntersechon, however, provides space for more than one vehicle waiting at the sup line side by side, e.g., vehicles performing more than one movement can use He stop line position at He same time. If k is defined as He number of spaces for passenger cars belonging to one movement that can queue at the stop line without obstructing the access to the stop line for other movements, it is clear that with k > 0, the capacity of the minor street approach is increased compared with the sharedIane oondidom Wig He increase of k, the total capacitor approaches He case that each movement has its own individual lane of infinite length.
|
From page 93... ...
I-his is equivalent to the average queue on the approach for the separate lane case. Derive a maximum length in vehicles ken of the flared area above which the traffic flows approximately like it would be on two separate lanes.
|
From page 94... ...
The more vehicles traveling in platoons, the higher the minor street capacity for a given opposing volume because there is a greater proportion of large gap sizes which more than one minor sweet vehicle can use. As shownin appendix ~ of the Highway Capacity Manual, the effect of upstream signals on both major street approaches is more complex.
|
From page 95... ...
However, the mode} did not predict field capacities as wed as the Harders or Siegioch models which assumed random arrivals. However, the concept of bunching is a useful descriptor of He effects of upstream circled stances on arrival headways.
|
From page 96... ...
The second shortcoming of the HEM 1994 method is that the signals on either side of the TWSC intersection are assumed to have the same cycle length, and be coordinated with a constant offset. This simplification means that at a given TWSC intersection location, the same major street arrival pattern is repeated each signal cycle.
|
From page 97... ...
A better assumption would be to use the ac~alplatooned volume, discharged at the saturation flow rate until the queue at He signalized approach has . dissipated.
|
From page 98... ...
= F2s-T + (1-F~Q,', (152) aigonthm used byTRANYST-7P signal analysis software (Yu and Van Aerde, 1995~: where I' is the vehicles arriving at the subject location in time slice t, Qt.
|
From page 99... ...
for each major street flow regime. Solving the second shortcoming of the current 1994 HCM method requires that the "begin platoon" and "end platoon" events from each major street direction be obtained for the whole analysis hour (direction 2 is major left, direction 5 is major nght)
|
From page 100... ...
Table 49 provides an aggregation of the percent increase In capacity for each movement compared to random arrivals sort by green time as columns, and signal spacing and distance to signal 2 as rows. For the same experiment, Table 50 provides an aggregation of the vehicular increase In capacity for the minor through movement compared to random sort by signal spacing, distance to signal 2 as columns, and green time, total approach vogue, and offset as rows.
|
From page 101... ...
Percentage Idcrcasc in Capacity As Function of Upstream Signals as Comb to Random Major Street Arrivals ~ ~ 2e2~ ~5~2-2~ ~ 5~ ~ 5~T ~J~5 ~ ~ ' ~ MinLT 68 78 84 MinTH 49 S6 S9 SOD MinllT 22 23 22 MajLT 9 10 9 0.2S MinLT 64 7S 84 MinTH 46 S4 60 1000 MinRT 13 13 12 MALT S S S MinLT 39 42 40 MinTH 28 31 29 SOO MinRT 22 23 22 MajLT 9 9 9 MinLT 33 34 3S MinTH 24 24 25 1000 MinRT 14 13 12 MajLT 6 S S MinLT 31 33 36 MinTH 22 24 26 lSOO MinRT 7 6 S O.S MajLT 3 2 2 MinLT 3S 41 46 MinTH 26 30 34 2000 MinRT 2 2 2 MajLT 1 1 1 MinLT 42 S2 S9 MinTH 32 39 4S 2SOO MinRT 1 1 O MajLT O O O MinLT 31 34 34 MiIITH 23 2S 2S SOO MinRT 22 23 22 MajLT 9 9 9 MinLT 20 19 MinTH 14 14 14 1000 MinRT 13 13 12 MajLT S S S MinLT 10 8 MinTH 7 4 6 1 lSOO MinRT 7 4 S MajLT 3 0 2 MinLT 4 3 3 MinTH 3 2 2 2000 MinRT 2 2 2 MajLT 1 1 1 MinLT 3 1 1 MinTH 2 1 O 2SOO MinRT 1 1 O MaiLT ~ O O O 77 SS 18 7 81 S9 9 30 22 17 7 28 20 34 25 4 49 36 o 66 50 o 27 20 18 7 14 10 9 6 4 4 1 2 1 1 o 1 1 1 o 77 SS 21 9 76 SS 12 5 38 28 21 9 33 23 12 5 34 24 2 43 32 2 5S 42 o o 31 23 21 9 18 13 12 S 8 2 3 2 2 1 o o Notes: (1) D2 - Di~to Uh;beam Signal on~e I~ Side of ~e Minor S~ Appn~acb, (ft)
|
From page 104... ...
impact of me Level of Platooning in We Opposing Plow on the Minor Street Capacity The above discussion indicates that it is Important In many urban situations to include the effect of upstream signals Intersections when determining the capacil~,r of adjacent TWSC Intersections. This was confined by a user survey during Phase ~ of Me project where users aIrnost n~mmously requested Mat a new pro - =e acknowledge the effect of upstream signals.
|
From page 105... ...
win be a constant time based on walking speed. Consider a major street left mining vehicle exiting the Intersection onto a minor street leg which pedestrians need to cross.
|
From page 106... ...
Let subscnpt: S denote crossing of We subject minor street approach O denote crossing of the opposite minor street approach ~ denote crossing of We major street to the leg of the subject minor sweet approach R denote crossing of the major street to the right of the subject minor street approach Ranking Pedestrian Movements. A decision also needs to be made regarding the appropriate ranldng of pedestnans between the vehicular movements.
|
From page 107... ...
For planning applications, the number of pedestrians could be defined as it is In the chapter on signalized Intersections: low= 50/hr moderate= 200~r high= 400/hr As is done wad major sweet nght turning vehicles which In some cases have a weight of 0.5, each of We four pedestrian movements could be included in conflicting traffic equations, with weights of I.0 for higher ranked pedestnan movements and weights of O for lower ranked movements, except for a proportion of lower ranked pedestrians in "non-compliance" with their ranking rules. In practice, pedestrian volumes would usually comprise only a small proportion of total conflicting volume.
|
From page 108... ...
While the delayed rank ~ vehicles are discharging from the queue formed behind a major left turning vehicle, they impede lower ranked movements with which t hey conflict. However, He current 1994 HEM does provide an impedance for major street left turn vehicles In a shared lane.
|
From page 109... ...
CONCLUSIONS This section has introduced both emp~ncal evidence and suggest theoretical models to adjust the basic capacity or delay equations to account for some common occurrences at TWSC intersect `~ two-stage gap acceptance; flared minor street approaches; effects of upstream signals; effects of pedestrians; and delay to major street rank ~ vehicles. Singly or In combination, these effects can cause significant adjustments to the basic capacity and delay models.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.