The trouble with traffic studies: why bad traffic predictions are making our cities worse and what courts should do about it

• Pages: 325-376
• Date: 01 October 2024
• Published date: 01 October 2024
• Author: Kenneth Stahl,Kristina Currans
• Subject Matter: Derecho Público y Administrativo

THE TROUBLE WITH TRAFFIC STUDIES:

WHY BAD TRAFFIC PREDICTIONS ARE MAKING

OUR CITIES WORSE AND WHAT COURTS SHOULD

DO ABOUT IT

Kenneth Stahl and Kristina Currans

1

Synopsis: Land use dec isions today al most invariably require predic -

tions about the amount of traffic a new development will g enerate, but

these predictions are often inaccurat e because the predomina nt

method used to pred ict traffic impacts tends to significa ntly over-

estimate vehicle traffic . As a result, land use decision s predicated on

such inaccurate traffic predict ions end up perpe tuating automobile

dependency and generating hostili ty to new housing deve lopment,

deepening a seve re housing crisis th roughout the country. Planners,

lawyers, developers, elected officials, a nd courts mostl y seem content

to ignore these obvious problems, but th eir complacency m ay not last

much longer. This Article argues tha t the con ventional, vehic le-

oriented me thod for predicting t raffic impacts does no t withstand

judicial scrutiny under a ny of the stan dards of review applicable to

land u se decisions. Therefore, we conclude that courts should review

prediction s of future traffic with greater skeptic ism. Indeed, the United

States Supreme Court recen tly decided an im portant case, Sheetz v.

County of El Dorado, California , 601 U.S. 267 (2024), that may require

courts to review traffic predic tions more closely. Our research should

also encourage members of the public to push b ack agai nst tra ffic

studies tha t over-estimate traffic impacts and spur agencie s to explore

and ado pt more reliabl e methods for estimating the traffic impacts of

new develop ment.

1

Kenneth Stahl is a Professor of Law and Director of the Environmental, Land Use

and Real Estate Law program at Chapman University Fowler School of Law. He is also a

practicing land use attorney who assists developers and housing organizations to obtain

approvals for housing projects in California. Kristina Currans is an Assistant Professor at

the University of Arizona School of Landscape Architecture & Planning. For helpful

comments, the authors thank participants at the Planning, Law and Property Rights

Conference at the University of Michigan Taubman College of Architecture & Urban

Planning, the State and Local Government Law Conference at Northwestern Pritzker

School of Law, the People for Housing Orange County webinar series, and the Chapman

University Fowler School of Law Faculty Workshop series, as well as three anonymous

peer reviewers. We also thank Cara Vincent Williams for excellent research assistance.

326 59 REAL PROPERTY, TRUST AND ESTATE LAW JOURNAL

I. INTRODUCTION .......................................................................... 326

II. THE USE AND MISUS E OF TRIP GENERATION AND TRAFFIC

STUDIES ..................................................................................... 332

A. Trip Generation and Traffic Studies ..................................... 334

B. The Prominence of Traffic Studies/Trip Generation and Their

Uses in Practice .................................................................... 335

C. Theoretical and Practical Issues with Traffic Studies ........... 337

1. Flaws in the ITE Method ................................................ 338

2. Reliability: What We Do and Don’t Know ..................... 341

3. Innovations .................................................................... 342

III. STANDARDS OF REVIEW ............................................................ 345

A. Legislative Decisions and the “Fai rly Debatable” Standard.. 347

B. Quasi-judicial Decisions and the Substantial Evidence

Standard ............................................................................... 353

C. Exactions and the Nexus/Rough Proportionality Test ........... 360

D. Special Benefit/Competent Evidence ................................... 363

IV. ADDRESSING OBJECTIONS ........................................................ 366

A. Generalist Courts Cannot Reasonably be Expected to Evaluate

Expert Testimon y with This Level of Granularity. ............... 366

B. Applying Heightened Scrutiny to the Conventional Method Would

Undermine the Ability of Agencies to Finance Needed

Services and I nfrastructure. .................................................. 368

C. Applying Heighte ned Scrutiny to the Conventional Method

Would have the Unintended Effect of Making Project Approvals More

Difficult. ............................................................................... 372

D. Given their Attachment to the Status Quo, It’s Unlikely that Any

Developer Will S ue to Invalidate t he ITE Method ................ 374

V. CONCLUSION ............................................................................. 376

I. INTRODUCTION

Virtually all land use decisions require rough predictions of future

traffic. Suppose a developer asks a city for approval to build a large

apartment complex on a busy commercial thoroughfare. Before

approving the project, the city will need to know how much automobile

FALL/WINTER 2024 THE TROUBLE WITH TRAFFIC STUDIES 327

congestion the project will add to already congested roads.

2

Or, suppose

a city is considering whether to change its zoning regulations to permit

religious institutions in a residential neighborhood to satisfy federal

requirements that cities’ land use rules accommodate religious land

uses.

3

A “rezoning” of this sort is like ly to have some impact on existing

automobile traffic, but the precise impact will depend on numerous

factors, such as the number of congregants expected to attend worship

services, how often and when such services take place, the mode of travel

congregants typically use to attend services, and so forth. In order to craft

an appropriate zoning ordinance that meets the need for new religious

institutions while respecting existing neighborhoods, the city must find

some way of estimating traffic impacts that takes account of these vari-

ous factors.

For years, cities and other planning agencies have conventionally

predicted vehicle traffic using “trip generation” statistics provided by a

non-profit organization called the Institute of Transportation Engineers

(“ITE”).

4

These are “off the rack” estimates of the number of trips

produced by a particular land use—5.44 trips per day for each apartment,

3.81 trips for every 1,000 square feet of a shopping center, and so on.

5

To determine how much traffic a particular project will generate,

agencies will commission a traffic study, or “Traf fic Im pact Analysis”

(TIA), which typically plugs these trip generation rates into the actual

land uses proposed by the project to produce a number of estimated

trips.

6

For example, if a project proposes 100 apartments, the traffic

2

In some places, local governments may be prohibited from approving projects that

cause traffic on nearby roads to exceed a certain threshold of congestion. See, e.g., CAL.

GOV’T. CODE § 66474 (requiring denial of subdivision map if the site is n ot “physically

suitable” for the type and density of development). Even if not, cities often face political

pressure from nearby residents not to approve projects that generate too much traffic. See

ANTHONY DOWNS, STILL STUCK IN TR AFFIC: COPING WITH PEAK HOUR TRAFFIC

CONGESTION 13–36 (2004).

3

Under the Religious Land Uses and Institutionalized Persons Act (RLUIPA), 42

U.S.C. § 2000cc(b), municipalities must provide areas where religious institutions are

permitted “as of right,” without any discretionary permits. Prior to RLUIPA, religious

institutions were often required to go through a discretionary approval process because of

the traffic impacts often associated with such institutions. After the passage of RLUIPA,

many cities have had to rezone areas of town to permit religious land uses as of right, as

described in the text.

4

See INST. OF TRANSP. ENG’RS, 2 ITE TRIP GENERATION MANUAL (10th ed. 2017).

5

See id. at 73, 140.

6

See, e.g. Scott Kelley, What is a Traffic Impact Study?, GREENLIGHT TRAFFIC ENG’G

(Feb. 5, 2024), https://www.greenlighttrafficengineering.com/blog/trafficimpact-Study.