KYOVA Metropolitan Planning Organization

TRAFFIC ASSIGNMENT

The traffic assignment phase deals with the trip-maker's choice of path between pairs of zones by travel mode and with the resulting traffic volume on the multi modal transportation network. The highway traffic volumes that will be assigned to the a highway segment depends on the capacity of the highway segment and whether the segment is a part of the route choice of trip makers. The traffic assignment models contain procedure to calculate the shortest path between zones and assign traffic to the network links. The basic premise is that the traffic will follow the route with least impedance, or in other words, with shortest travel time. However, when the volumes on the a route or link approach the capacity of the link, speed is reduced and trips begin to be diverted to other routes which are now faster. This method of assigning traffic to a network is called "capacity restraint" and represent the choices drivers make when driving on the actual highway network. The HIATS travel demand model uses an equilibrium assignment algorithm as part of the capacity restraint program. The equilibrium algorithm allows for the assignment of intrazonal traffic multiple paths based upon the intrazonal travel times. The output of this phase is assigned traffic volumes on the highway network.

Using the highway network and the highway vehicle trip table, the QRSII load highway network function is used to load the highway network. A Bureau of Public Road (BPR) incremental assignment technique will used different load percentages .

MODEL SPECIFICATION

The assignment methodologies are determined by the structure of the network, available path-building algorithms, and capacity restraint capabilities.

IMPEDANCE

The highway network characteristics contain data to determine the travel impedance for each path or route, where travel impedance is defined by some combination of travel time and cost. The travel impedance is explained in chapter 4.

The value of speed used in calculating travel impedance represents average observed uncongested speeds identified as "free-flow" speeds. The application of the trip assignment results in an estimate of congested speeds.

CAPACITY

The capacity of a roadway link is affected by level-of-service on the link. The capacity of a freeway link at level-of-service "E" may be 2000 vehicles per hour per lane, when the capacity of the same freeway link at level-of-service "C" might be 1,750 vehicles per hour per lane. At KYOVA, the travel demand model uses link capacity defined at level-of-service "C". The capacity will impact the congestion on the link, defined by volume-to-capacity ratio, and also the delay on the link, caused by congestion.

STATE-OF-THE-PRACTICE VOLUME -DELAY RELATIONSHIPS

The state-of -the practice in traffic assignment uses link based volume-delay functions. The variables that control the final assigned travel speeds, the beginning or free-flow speed, and the link capacity are link based. Typically, free flow speeds and link capacities are determined via a look-up table that relates these variables to the facility type or functional class of the link and the area type surrounding the link. As an example the look up table of free-flow speeds and per lane link capacities used for Huntington traffic demand model is shown in table Below. Such lookup table approach was used in the Urban transportation Planning Software (UTPS) distributed by the Urban Mass Transportation Administration in the 1970s and 1980s and, and as a result, has become a commonly used approach to estimating link-specific, free-flow speeds and capacities.

Table: Look-up Free Flow Speeds and link Capacities

Area Type		Functional Class Freeway	Functional Class Ramp	Functional Class Principal	Functional Class Minor	Functional Class Collector
CBD	Capacity	1,500	700	1000	700	600
Urban	Capacity	1,500	700	1000	700	600
Suburban	Capacity	1,800	800	1,200	800	700
Rural	Capacity	1,800	800	1,200	800	700
CBD	FF Speed	55	25	45	35	25
Urban	FF Speed	55	25	45	35	25
Suburban	FF Speed	65	35	55	45	35
Rural	FF Speed	65	35	55	45	35

In addition to the use of look-up tables to estimate link specific, free-flow speeds and capacities, the Bureau of Public Road (BPR) equation is used to estimate the effect of existing traffic on travel times or delay. The new estimated travel times or delays are employed to find the minimum travel paths for the second and third assignment iterations of the "capacity restraint" assignment technique. The BPR equation is shown below:

T=T_o*[1+0.15*(V/C)⁴]

Where, T = New travel time

T_o= Original or previous travel time

V = Volume assigned from first or second assignment iteration;

C = Capacity at level of Service "C".

MODEL VALIDATION

The purpose of the model validation is to ensure the accuracy of the whole travel forecast model. The basic goal in validating the travel demand model is verify that the 1990 assigned auto vehicle traffic volumes simulate the counted volumes on the actual roadway network for the same year. This is an iterative process in which the assigned volumes are compared to actual ground counts. If the results do not correspond to the counted volumes, adjustments are made to the model in a number of ways. A region-wide problem might indicate the need for adjusting trip rates in the trip generation model. Over or under-assignments in specific locations might require reviewing and adjusting socioeconomic data at the TAZ level. Problems at the link level might indicate the need to adjust the capacity, speed or other link attributes of the links with problems. This iterative process was repeated until the assigned volumes accurately simulated the ground counts for the study area.

The FHWA has developed statistical targets for measuring the level of validating of a travel demand model. The HIATS model was refined until met many of FHWA guidelines. .

Table : Calibration Target Summary
Assigned Volume-Mile-traveled By Facility Classification

FC-Rural	Model	Required*	Percent Difference	Req�d Percent Difference
1	636,809	607,342	+4.9%	7.0%
2 6	1,121, 797	1,019,080	+10.0%	10.0-5.0%
7 8 9	1,930,674	1,828,275	+5.6%	25.0%
Total	3,689,262	3,454,697	+6.7%
FC-Urban
11 12	1,133,703	1,187,906	_4.5%	7.0%
14 16	1,621,403	1,928,727	-15.9%	10.0-15.0%
17 19	900,660	774,117	16.3%	25%
Total	3,655,766	3,890,748	-6.0%
TOTAL	7,345,028	7,345,495	0.0%	3%

Table: Calibration target Summary
Assigned Volumes by Volume Group

Total ADT	Model	Tolerance
0 � 999	95.9%	72.3%
1,000 � 2,499	82.9	57.8
2,500 � 4,999	54.5	48.9
5,000 � 9,999	30.5	40.9
10,000 � 19,999	28.6	33.9
720,000	20.2	27.7

An additional measure of model accuracy can be achieved by reviewing 6 screenlines and 24 cutlines in the study area model. A screenline is an imaginary line drawn through the area which divides the study area into two parts. The volume of traffic crossing that line in the model is then compared to actual ground counts. This gives a broad indication of whether the overall volumes in the model are approaching the correct values for the area. Six screenlines were developed for the Huntington Metropolitan Area and the assigned traffic volumes were well within the FHWA calibration standards. Four of the six screenlines are within the target error range and two are outside of the range, but have an error that within 10 percent. These two screenlines have relatively low volumes and the error acceptable for making improvement decisions.

Additionally, cutlines were developed for the major corridors in the area. Cutlines are short screen which give a more accurate indication of whether the volumes in a specific corridor are within acceptable parameters. Twenty-eight corridor cutlines were developed and most of the cutlines volumes were within FHWA standards or just outside of the target range( ten are within the range and nine are within fifteen percent of the counts)when compared to existing ground counts. With the exception of three cutlines that have relatively modest, the remaining six cutlines have relatively low volumes and the error is within acceptable limits for these low volume roadways. Table 5.4 shows the screenline/cutline analysis results.

Table : 2000 Validation Traffic Assignment External Cordon,
Screenline and Cutline Stations Summary

Screenline	Location	Model	Ground Count	% Diff.
1	US52 WV527 WV106 Total	22,851 14,067 15,563 52,481	22,000 15,000 16,500 53,500	-1.9
2	Total ADT	Model	Tolerance	-3.7
3	0 � 999	95.9%	72.3%	-6.7
4	1,000 � 2,499	82.9	57.8	+4.6
Bridges	2,500 � 4,999	54.5	48.9	-3.8
Total	5,000 � 9,999	30.5	40.9	-1.1
	10,000 � 19,999	28.6	33.9
	720,000	20.2	27.7

SUMMARY

Most of the FHWA urban model calibration targets have ben met by this model. The trip generated model produced expected trip volumes, the trip distribution model resulted in average trip lengths comparable to other travel studies and traffic assignments produced volumes reasonably within FHWA standards. Therefore, the model can be used in forecasting 2030 traffic to examine future year capacity deficiencies and perform future year plan development and analysis. Future year analysis required the substitution of the base year socioeconomic data with the 2018 future year data and then the entire modeling process was rerun to develop future year values.

ANALYSIS OF TRAFFIC VOLUMES

Several types of analysis can be performed using QRSII system.

V/C analysis

The load highway network function is used to print ratio of volume to capacity (V/C) for each link. These ratios indicate the level of congestion in the highways.

System Deficiencies

Traffic assignment will be performed by using socioeconomic data on the 1990 network to identify system deficiencies.

RECOMMENDATION

As a result of this study, a number of recommendation are listed below to help improve future validation and calibration efforts of travel demand model.

1. New household travel survey

the current travel models were developed based on the data results collected back in 1971. However, many changes have taken place in 31 years between 19971 and 1998 which necessitate the need to re-evaluate the travel models for accuracy based on updated travel survey data. Since 1971 HIATS has seen a continued dispersion of residence and employment from older urban areas to the periphery accompanied by a shift from suburban to city to a predominated suburban to suburban travel orientation.

2. Develop HBW Trip Length Frequency Distribution

This development should use the 2000 CTPP adjusted work trip table and the 2000 gravity model skim tree results.