1Chapter 9-4e

Chapter 9. Volume Studies & Characteristics

Understand that measured volumes may not be true demands if not careful in data collection and explain the relationships between volume, demand, and capacityExplain typical temporal and spatial variations of volume (from your reading)Set up a plan for continuous and coverage counts given a set of scheduling conditionsExplain the types of volume counts (control count, coverage count, etc) (from your reading)Determine daily, weekly, monthly, and seasonal count adjustment factors

Chapter objectives: The student will be able to (we spend 2 lecture periods for this chapter):

Some of the topics in this chapter were covered in CEEn 361. Please review those topics by yourself.

9.1 Critical Parameters

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VolumeRate of flowDemandCapacity

There are four variables related to volume:

Can you explain the differences among these volume-related variables?

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9.2 Volume, Demand, and CapacityVolume: The number of vehicles (or persons) passing the measurement point during a specified time interval. “What is”

Rate of flow: The rate at which vehicles (or persons) pass a point during a specified time period less than one hour; expressed as an equivalent hourly rate.Demand: The number of vehicles waiting for service in the given time period, as distinct from the number that can be served. Capacity: The maximum number that can be expected to be served in the given time period. “What is possible”

Watch out! Depending on the situation, volume may not reflect demand.

Upstream bottleneck capacity restrict the flow downstream.

d = 3800+3600=7400>6000

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Volume, Demand, Capacity (cont)

When capacity constraints exist upstream, demand cannot be measured downstream, because the discharge volume is NOT demand but bottleneck capacity volume.

May be mistaken as demand

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Fig 9.3 Effects of a bottleneck on observed volumes

3400x(500/3700) = 4593400-459=2941(Assume the proportion of off-ramp demand is the same for 3700 and 3400 vph.

2941x(400/3200) = 3682941-368=2573(3700-500 = 3200). Assume the proportion of off-ramp demand is again the same for 3200 and 2941.)

3700

3700 – 500 – 400 = 2800

9.3.1 Hourly Traffic Variation Patterns: the Peak Hour

6Chapter 9-4eUDOT Signal Performance Metrics

At I-15 Point of Mountain

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http://udottraffic.utah.gov/signalperformancemetrics/Access to UDOT’s Signal Performance Metrics:

Fig 9.5: Peak Hour as a % of AADT

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K Factor

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9.3.2 Subhourly variation patterns: Flow rate vs. volume

Figure 9.6

PHF = Hourly Vol/(4 * Volpeak 15)

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9.3.3 Daily Variations

Figure 9.7

9.3.4 Monthly or Seasonal Variations

11Chapter 9-4e Figure 9.8

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9.4 Intersection Volume Studies

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9.4.1 Arrival vs. departure volumesUsually discharge volume is counted at intersections. But when demand > capacity, queue is formed and discharge volume is never greater than the capacity. Hence, adjustments must be made to discharge counts to estimate demand when queue is formed.

Counted at the beginning of each red phase at a signalized intersection: Why is this counted at the beginning of each red phase (interval), or at the ending of each yellow phase?

For unsignalized intersections, the queue is counted at the end of each count period.

9.4.2 Special Considerations for Signalized Intersections

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Figure 9.9

Both the counting and the short-break periods must be equal multiples of the cycle length when you count at signalized intersections. See the example in the left column of page 176, Section 9.4.2.

If cycle length is 60 sec, 15 min counting (15 cycles, 14+1) is good. For a 90-sec cycle length, 15 min (10 cycles, 9+1) is again OK. For 120 sec cycle length, 16 min (8 cycles, 7+1) is good.

(Try prob. 9-3)

The assumption of the above values is that you have one cycle of rest time. With JAMAR you can continue counting.

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9.5 Limited Network Volume StudiesSmall-network volume studies are intended to determine the amount and pattern of traffic flow over a limited network of street links and intersections during a specified interval.

Two types of counts are needed:

The extent of counting depends on resources -- money and manpower. Usually a sampling technique is used.

9.5.1 Control counts

At control count stations, volumes are counted throughout the study period. (It’s like a permanent count, but only for the study period. Read the left column of page 178.)

9.5.2 Coverage counts

Any count stations other than the control count stations have sample counts taken. Volumes counted at coverage stations are adjusted by the data from control count stations.

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Example of a small-network volume study:An 8-hr study from noon to 8 pm(pp. 179-184)

Control count STA

Coverage count STA

One-day (8-hr) study: Needs two crews: one at the control count station (8 hrs) and the other at coverage count stations (1 hr at each coverage station).

Multi-day (6-day) study: Needs two crews: one at the control count station (8 hr/day, 6 days) and the other at coverage count stations (8 hr at each coverage station).

Combined (3-day) study: Needs two crews: one at the control count station (8/hr/day, 3 days) and the other at coverage count stations (4 hr at each coverage station, 2 stations per day).

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6

4

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5

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One-day Network Volume Study p.180.(Assume a similar peaking at all stations.)

Peak Hour has 16.3% of the total 8-hour count.

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6-day Multiday Study Option, p.181-182

Fvi = Va/Vi

Va = ViFvi

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Tree-day Study Option, p.181 &183

Don’t seethis column.

Estimating Vehicle Miles of Traveled on a Network

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VMT values are used for many analyses, like crash rate determination and air pollution estimation.

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Some general principles for statewide programs are:1. Objective: Conduct a coverage count every year on every

2-mile segment of the state highway system except low-volume (AADT<100 veh/day) roads.

2. Objective of coverage counts: Produce an annual estimate of AADT for each coverage location (then VMT)

3. One control-count location for every 20 to 50 coverage-count locations (stratified highway groups)

4. Control-count location can be either permanent counts or major or minor control counts. Control count stations count 24 hours 365 days to determine adjustment factors

5. All coverage counts are for a minimum period of 24-48 hours, eliminating the need to calibrate hourly variation patterns.

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9.6 Statewide Counting Program

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UDOT’s permanent count stations.

http://www.udot.utah.gov/main/f?p=100:pg:0::::V,T:,528

UDOT Traffic Maps:

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9.6.1 Calibrating Daily Variation Factors

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9.6.2 Calibrating Monthly Variation Factors

From permanent count stations.

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9.6.4 Using the Results

365365 LAADTVMT

AADT (Tue, July) = 1,000 * 1.121 * 0.913 = 1,023 vehs/day

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9.7 Specialized counting studies (covered by CEEn565)

9.7.1 Origin & destination counts

For regional planning, home-interviews are needed. In traffic applications, the scope of origin and destination counts are more limited (e.g., get on an on-ramp and get off at an off-ramp downstream).

9.7.2 Cordon counts

When vehicle accumulation within an area is needed (See Tab 6-12)

9.7.3 Screen line counts

When changes in traffic flow from one area to another is needed

OD count is discussed in detail in CEEn 565.

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8.7.1 Origin and destination counts (covered by CEEn565)

Conventional traffic origin and destination counts rely primarily on one of the three approaches:

• License-place studies• Postcard studies• Interview studies

If you have a lot of transponders uses for toll roads, you can automatically collect OD (on-ramp/off-ramp combinations).OD analyses need a lot of iterations.

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(Discuss the example in Figure 8.19 and Table 8-14.)

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8.7.2 Cordon counts (covered by CEEn565)

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