Simpson County Travel Demand Model

Mobility Analysis

November 7, 2003

Study Study LocationLocation

MODEL BACKGROUND

•The main objective was to forecast traffic volumes on a new section of the KY 1008 bypass in Franklin, KY

•Project was coordinated through:

• The Division of Planning

• The Division of Multimodal Programs

•KYTC decided to build a full travel demand model for Simpson County for future uses such as:

• Air quality analysis (non-attainment)

• Any other transportation-related testing

BACKGROUND (CONT.)

• Additional purpose was to test mobility in Simpson County

•KYTC wanted to apply Texas Transportation Institute’s (TTI) Mobility Indices in a travel demand model

•The result was a preliminary set of procedures that could be used to quantify mobility using travel demand models

RESEARCH

• Two reports were reviewed as part of this project:

• The 2002 Urban Mobility Study (TTI):

• Outlines the definitions and procedures for determining mobility indices

• Includes results of indices throughout U.S.

• A case study of Grand Junction, Colorado:

• Written by TTI and the Colorado Department of Transportation

• Uses travel time research to derive area-wide mobility indices

2002 URBAN MOBILITY STUDY

• Methodology can be found in Appendix B of the 2002 Urban Mobility Study report

• Other information included in report:

• Constants

• Formulas

• Sample Calculations

• Mobility Indices of Major Cities in U.S.

GRAND JUNCTION CASE STUDY

• Grand Junction was used to test TTI’s mobility methodology in the year 2000

• Travel time was most important attribute for accurate results

• Travel Time Data was collected during the following periods:

• AM Peak

• PM Peak

• Off Peak (Free Flow Period)

GRAND JUNCTION CASE STUDY (cont.)

• Additional data collected included:

• Road segment distance

• Vehicle occupancy

• 24 hour traffic counts

SIMPSON MODEL ISSUES

• The KYTC wanted the TTI methodology to apply to travel demand models

• Much of the data collected can be obtained from a travel demand model

• However, the Simpson County Travel Demand Model was a 24-hour model and did not contain peak volumes

• Initially, it was believed that not having the peak hour traffic information may limit the use of the TTI methodology

ADDITIONAL INFORMATION

• The TTI methodology applies to Interstates and Principal Arterials

• Since there were not any Principal Arterials in Simpson County, Minor Arterials were used as the next ‘best’ thing in the analysis

• Roads such as I-65, US 31W, KY 73, KY 100, KY 383, and KY 1171 were used

TTI INDICES

• RCI – Roadway Congestion Index

• TRI – Travel Rate Index

• TTI – Travel Time Index

ROADWAY CONGESTION INDEX (RCI)

• Provides an indication of the total number of hours in a day that a road may experience congestion

• Therefore, a value of 20% would indicate that 20% of the daily travel along the road occurred in congested conditions

• Also, the RCI can be used to determine the annual person-hours of delay for a specific study area

RCI INPUTS

• The index requires the following input:

• Number of Lanes

• ADT

• Peak Directional Traffic

• Speed Estimates

• Estimates of Travel Delay

RCI PROBLEMS / SOLUTIONS

• As previously noted, the Simpson model did not include peak hour directional forecasts

• Because of this, a ‘true’ RCI calculation could not be calculated based on TTI methodology

• However, a similar index could be calculated by:

• Subtracting modeled travel time from free flow travel time

• Multiplying result by number of vehicles on segment

• This was conducted for all interstates and arterials

RCI RESULTS

• Based on this procedure, the average annual delay in Simpson County was 0.87 hours per person.

• In the 2002 Urban Mobility Study, the smallest RCI value was 5.0 hours/delay per person in Brownsville, Texas

• Considering Brownsville, Texas is nearly ten times the size of Franklin, KY, the value for Simpson County seemed reasonable

TRAVEL RATE INDEX (TRI)

•Provides an indication of the total amount of extra time required to make a trip as a result of congestion along a roadway

•Therefore, a value of 1.20 would indicate that it would take 20% longer to make a trip during peak periods when compared to free-flow speeds

TRI INPUTS

• The index requires the following input:

• Average Freeway Speed

• Freeway Vehicle Miles of Travel

• Average Arterial Speed

• Arterial Vehicle Miles of Travel

TRI FORMULA

Travel Rate Index =

FreewayTravel Rate

FreewayFree Flow Rate

Peak PeriodFreeway VMTx +

Prin. ArterialTravel Rate

Prin. Arterial

Free Flow Rate

Peak PeriodPrincipl

e Arterial

VMT

x

Peak Period

Principle Arterial

VMT

+Peak PeriodFreeway VMT

TRI PROCEDURE

• The following steps were taken to calculate TRI:

• Calculate average model speed for each segment

• Calculate VMT for each segment

• Calculate a Free Flow Rate per segment

• Assume a K-Factor to obtain a peak VMT per segment

• Use TRI equations to calculate study area TRI

TRI RESULTS

• Based on this procedure, the Travel Rate Index was calculated to be 1.00256 hours per person

• This indicates that it will take approximately 0.2% longer during peak periods than free flow periods

• Based on the TTI report, Anchorage, Alaska and Corpus Christi, Texas had the lowest TRI value of 1.02

• Therefore, a value of 1.00256 in Franklin, a much smaller city, seems reasonable

Future 2025 TRI

RESULTS

Scenario DescriptionFreeway

TRIArterial

TRIOverall

TRI

1 Base 2002 1.00 1.00 1.00

2 Base 2025 (I-65: 4 Lanes) 2.92 1.89 1.92

3 E+C 2025 (I-65: 4 Lanes) 2.18 1.15 1.17

4E+C 2025 with Bypass

(I-65: 4 Lanes) 2.18 1.07 1.09

5 Base 2025 (I-65: 6 Lanes) 1.04 1.68 1.66

6 E+C 2025 (I-65: 6 Lanes) 1.04 1.09 1.09

7E+C 2025 with Bypass

(I-65: 6 Lanes) 1.04 1.09 1.08

TRAVEL TIME INDEX (TTI)

• The TTI is similar to the TRI, but more complex

• This index includes recurring and incident congestion whereas the TRI only considers recurring congestion

• The TTI also requires peak direction information to determine mobility

• Without this information, it would be difficult to use the TTI procedures

• As a result, it was decided that this index would not be included as part of the Simpson County analysis

Geographic Application of Geographic Application of Mobility Index MeasuresMobility Index Measures

In these illustrations, mobility indices for TAZ’s are used to illustrate the effect that a new bypass has option has on improving the mobility service for some areas and not affecting others.

0.00 is Best

>1.00 is Worst

New Bypass

MobilityImproved

MobilityUnaffected

CONCLUSIONS

• The RCI and TRI are two indices that can potentially be used to report mobility from a small-urban travel demand model

• Accurate speed data (peak periods) from the model is necessary for good results

• The mobility indices will produce more accurate results if used on a model with hourly assignments

• Consider opportunities for modified approach to deal with small urban areas

• Consider applications to correlate mobility indices to smaller geographic units (TAZ’s)

QUESTIONS?

COMMENTS?