Lec 29: Ch3.(T&LD): Traffic Analysis – Non-site traffic forecast

Understand why estimating non-site traffic forecast is necessary

Know three principal methods of projecting non-site traffic

Learn the advantages and concerns of the three principal methods

Why must we estimate non-site traffic?

Estimates of non-site traffic in the horizon year characterize the “base” conditions for the proposed development What would be the traffic in the study area without the subject site being developed?

Components of non-site traffic:

Through traffic consisting of all movements through the study area, without OD in the study area

Traffic generated by all other developments in the study area, with OD in the study area (i.e., trips which do not have an origin or destination on the site of the proposed or existing development)

Methodology

Build-up method using specific development

Use of area transportation plan or modeled volumes

Trends or growth rates

Estimate the “base” condition in the horizon year.

Advantages and concerns: 1. Build-up method

Typically appropriate in areas of moderate growth

Usually used when project has horizon of 10 years or less

Often best method when there is good local information on development approval

Advantages and concerns: 2. Use of transportation plan

Often used with large, regional projects that will develop over a long period

Often appropriate for areas of high growth

Locally credible transportation plan data that is adaptable to the study year must be available

Advantages and concerns: 3. Growth rates or trends method

Typically used for small projects that will be built within a year or two

Local record keeping of traffic counts must be good

At least 5 years of data showing stable growth should be available

Simple, straightforward approach

Not appropriate for long-range horizon

May result in over- or undercounting non-site traffic growth

Build-up method

Appropriate when considering a study area with several major projects being developed during the same period as the subject project

Most beneficial when desired to accurately reflect local area traffic patterns that will change as a result of limited or moderate non-site development or that are in an area where available traffic volume projects are sketchy

Build-up method: steps

1. Assess impacts of committed system improvements during forecast period

2. Identify study area development within forecast period

3. Estimate trip generation by build-ups

4. Estimate directional distribution5. Assign traffic

6. Estimate growth in through traffic (This needs the other two methods)

7. Sum study area and through traffic

8. Check for logic of results and adjust as necessary

Use of area or sub-area transportation plan or modeled volumes

Typically an area-wide transportation plan model is concerned with major roadways and is meant for really aggregated values. The object is to find trends

None of the long-range forecast methods employed are intended to be accurate for the level of detail needed for intersections analysis Volume (especially turning volumes) may not be accurate at all.

Watch out!

Often turning conflicts are accounted for by using surrogate values like “turning penalties”.

(We skip the Graphical method in pages 65-70.)

Trends or growth rate method Assumption: Recent

growth rates in traffic volumes will continued through the study target year, or will change predictably both inside- and outside of the study area

Normally should not be used for horizons beyond 10 years due to the variability in growth rates over time and the magnitude of error that may result due to a relatively small error in the growth rate over a longer period of time.

0.5%

10%

6%

2 4 6Year

0Traffic year n = (Traffic year 0) x (1 + GF)n

Case studies from p.70-78

Read these two case studies carefully to understand the general procedures of TIA’s.

(We will cover these case studies in class when time permits.)

General guidelines

Check for the reasonableness of the proposed final projection of non-site traffic

When the build-up or transportation plan volumes are used, assess the percentage increase on critical street segments assess if growth rate is reasonable

It may be that the impact of an increase in non-site traffic is greater than that generated by the site under study this will affect cost-sharing with the local government Estimating non-site traffic for the horizon year is the most critical single factor in determining roadway improvement needs

Be ware of and consider the programmed or planned transportation system changes (which are highly likely)