(Disclosure: Summit Land Management is a Rose Law Group-related company, and contracted with the firm.)
By Paul Basha, Kayla Amado | Rose Law Group Reporter
In a word, EXTREMELY!
“Traffic” is one of those heavily burdened words. Hardly anyone associates that word with joy or
happy or glad or anything positive. Yeah, if others are having too much fun at a party, say “traffic”
and that will instantly turn things at least sorrowful or somber, usually angry. Happens to me all
the time.
Some people in our society really like traffic. It is our mission and livelihood. Our contribution to
bettering society. For us, traffic is numbers, data, and calculations – the epitome of human
existence. Like the motto of the children’s magazine, Highlights, “Fun with a Purpose”.
For a proposed development, every public agency requires a traffic investigation of some type. A
“Traffic Impact Study” or “Traffic Impact Analysis” for large proposed developments, or a “Traffic
Statement” or “Traffic Impact Statement” for small proposed developments. Most jurisdictions
define small as generating fewer than 100 vehicles-per-hour in the peak hour. Traffic Impact
Study and Traffic Impact Analysis are synonymous. The larger the development, the more
extensive the required Traffic Impact Analysis.
While different jurisdictions have different requirements, often, if a development is anticipated to
generate 100 to 500 vehicles-per-hour, then all intersections within one mile must be analyzed,
for the completion year of the development. Also, often, if a development is anticipated to
generate more than 500 vehicles-per-hour, then all intersections within one, two, or three miles
must be analyzed for the completion year of the development; and for five, ten, or fifteen years
into the future.
Generally, communities in the rapidly developing further out suburban parts of metropolitan
Phoenix have more stringent requirements. Communities in the more developed parts of the
Valley of the Sun generally have less stringent requirements. Though that is not always true.
Often, the greater the adjacent resident opposition to a proposed development, the more
demanding the public agency analysis requirements.
Frankly, the more opposition to something, the more scrutiny it should receive. Our government
works best by listening to all involved people and providing all interested people all information
on a specific topic of concern. We are a community-based system of government. Each of our
governments serves all residents, property owners, and travelers within its jurisdiction. The
governmental decision-makers listen to all of us and then decide. The government does not
dictate our daily lives or how we use property that we own (without due process).
Speaking of government dictating people’s lives and how we use our property. A few years back,
I was asked by Arizona State University College of Sustainability and the Built Environment to
substitute for a full-time traffic engineering professor for the second half-semester of one of his
courses. He had been asked by a government agency in another country to go there for several
months to help direct the future transportation planning efforts of a very large and growing
existing municipality. He and I chatted about the difficulties of planning the location and size of
streets for future development within existing cities. I asked him about the problem of acquiring
land from existing properties full of buildings. He responded that in the country he would be
assisting, they do not have that difficulty, the government owns all land. The government simply
gives the existing building tenants, either residential or commercial, not-quite-adequate notice to
leave, then the government demolishes the buildings, and builds the roads where and how large
the government decides.
Our system is so much better. I am so grateful that I live in a country governed of, by, and for
the people.
Private property rights are multi-faceted. People who live near a proposed development have the
opportunity to publicly provide their opinions and perspectives on how adjacent property should
be developed. A proposed project might affect their lives. Simultaneously, the owner of the
property proposed for development has the right to use their property as they choose.
Obviously, this dichotomy is frequently an inherent conflict. We democratically elect
representatives to decide between the differing, often opposing and contradictory, private
property rights. Occasionally these desire-balancing decisions are easy, frequently they are very
difficult. Intelligent and well-intentioned people, who care deeply for their communities, can have
very different opinions on the proper use of someone’s private property.
Repeatedly, the word, “traffic” is used as a hostile hopeful epitaph during the public discussion
of proposed developments. People who live near vacant property correctly recognize that a new
development on the existing vacant property will generate more traffic than the zero traffic
generated by the vacant property. It is inherently unfair for other people to demand that an
owner of vacant property keep it vacant forever, so that it generates no traffic. Only in
dictatorships do some people have the power to unilaterally control how other people use the
property that they own.
Often the question of whether or not a proposed development should occur depends on the
acceptability of the increased traffic. It is inherently unfair for adjacent property owners to
demand that a public agency restrict a particular property owner to someone else’s definition of
acceptable traffic.
The challenges are determining how much new traffic is acceptable, and the definition of
acceptable traffic. Objective, comprehensive, factual analysis of traffic is necessary to assist the
elected representatives with their deliberations and decisions. Fortunately, there are people with
this specific expertise available who can provide objective, comprehensive, and factual traffic
analyses. Some of my best friends know such a person.
The foundational determination for proposed development traffic impacts is trip generation. How
much traffic will the proposed development generate? Once we know that number, we can
collectively decide if it is too much or an acceptable amount of traffic for a specific location. We
can also collectively decide what improvements are necessary to accommodate that amount of
additional traffic.
Two sayings I often present in class to both undergraduate and graduate traffic engineering
students: (1) Please do not believe everything you think. (2) You know more than you think you
know, and simultaneously you know less than you think you know. Each of us has the right to
our opinions, so we should have self-confidence. Also, each of us should have the humility to be
sincerely curious and respectful of the opinions of others.
In addition to opinions, facts exist.
Estimating future traffic generated by proposed developments is a science. Back in 1976, the
Institute of Transportation Engineers (ITE) developed the first edition of the Trip Generation
Manual. This effort was initially prepared primarily by one person, Carl Butke, a traffic engineer
from Portland, Oregon. He surmised that we could predict traffic volume for a future proposed
development by counting the traffic at existing similar developments. Estimating future traffic
volume based on actual traffic counts at similar developments has been continuously utilized and
refined in North America for the past 50 years.
Recent editions of the Trip Generation Manual incorporate non-private-vehicle traffic comprising
bicycle, pedestrian, bus, or rail travel. As we live in a very car-centric metropolitan area and
country, the dominant focus of Traffic Impact Analyses is private motor vehicle travel.
In August of 2025, the Institute of Transportation Engineers published the 12th Edition of the Trip
Generation Manual. The Trip Generation Handbook, 3rd Edition, describes how to utilize the Trip
Generation Manual. (For years and currently, I’ve been selected to be one of a few dozen
professionals from throughout North America to edit and review these references prior to their
publication.)
The Trip Generation Manual has traffic count data obtained from 1990 through 2025 throughout
the United States and Canada from 967 sources for 182 separate land uses grouped into 10 land
use categories. Over the decades, we have removed old data from the Trip Generation Manual.
The editions we used until the 2000’s had traffic data from the 1920’s!
The idea is that larger developments – greater building area, more employees, more homes, more
residents, and such – generate more traffic than smaller developments. So, we examine the data
from existing developments, discover a rate of trips-per-hour (or trips-per-day) per 1000-squarefeet of building, or per employee, or per home, or per resident. We then use that trip generation rate and multiply it by the size of the proposed development.
In the past, our data for residential communities was separated by whether the homes were
rented or owner-occupied. For the past ten years or more, the data has revealed that there is no
difference in traffic generation between renters and owners, for either single-family or multifamily homes. This is fact, not opinion.
Data for two separate categories of single-family homes are provided: detached or attached. Data
for three separate categories of multi-family homes are also provided, depending on the height
of the buildings. Also, for the multi-family categories, the data are separated by close or not close
to transit.
For example, current data reveals that a single-family house generates an average of
approximately 9 trips-per-day and approximately 1 trip in the peak hour (both total of both
directions). This trip generation is all vehicles: people who live there, plus delivery people, repair
people, maintenance people, or other people. Current data also reveals that a two-story or three-
story apartment or condominium home generates an average of approximately 6 trips per day
and approximately 0.6 trips in the peak hour (again, total of both directions). Again, this is fact,
not opinion.
Initially, 9 total trips-per-day for a single-family home appears excessive. Also, peculiar that a
single-family home would generate more traffic than a multi-family home. However, this is all
traffic, not only residents, and for all purposes. So, work and shopping trips twice (leaving and
returning) each time for each person are included in the 9 total trips-per-day. Plus, recreation or
social trips, plus visiting guests; again twice for each event. Also, delivery and service vehicles
are counted, twice per visit.
By definition, multi-family developments have a much greater density of people than single-family
homes. Delivery and service vehicles serve multiple homes per trip for an apartment or
condominium complex property. Delivery and service vehicles typically serve one single-family
home at a time. How many of us would accept a plumber or Amazon driver telling us that they
will visit our house as soon as ten of our neighbors need the same attention? Even if several
single-family homes are on a delivery or service route, these homes are much further apart than
multi-family homes, which means more driving distance, which means a longer time of vehicles
on the road.
Recognize that the 9 or 6 trips-per-day is an average. Each of us travels different numbers of
daily trips each day, depending on numerous factors. Some of us travel to and from work seven
days each week; some none; most between seven and none, and most of us on the same four
or five days. Few of us grocery shop or visit other businesses every day. Few of us go to religious
services or athletic events or musical performances or movies every day. Many of us leave home
for restaurants some days, though few of us for all days. Importantly, every day, some of us
travel; and typically, every day some of us travel several times, with the same number of trips
away from home as returning home. (Obviously for extended days of travel, the return trips might
be on different days than the original trip.)
Almost always, there are many more homes in an apartment or condominium complex, than
homes in a single-family neighborhood. Single-family homes are typically two homes-per-acre to
six homes-per-acre, sometimes one home-per-five-acres or per-ten-acres. Low-rise and mid-rise
multi-family homes are typically eight homes-per-acre to fifteen homes-per-acre. Therefore, a
typical apartment complex generally generates more traffic than a typical single-family
neighborhood, though the amount of traffic depends completely on the size of the residential
communities. However, widely-spaced single-family homes need more miles of roads than multifamily homes.
Often, people believe that multi-family homes generate more traffic than commercial properties.
This belief is factually false. The opposite is true. A typical commercial development generates
four to twelve times as much traffic as a typical multi-family complex on the same property,
depending on the time of day, for all non-sleeping hours of the day. Candidly, commercial
properties need to sell products, which usually requires visitors traveling to their properties. Yes,
increasingly, commerce occurs through online shopping. Though the delivery person needs to
drive to and from your home, and to and from a store of some type, as well as from their home
to their work and from their work to their home. The factual data reveals that retail properties
generate approximately 17% less traffic in 2025 than in 2003. Pointedly, if a retail business did
not attract people (almost always in vehicles), the retail business would close. So, conversely, if
the retail business exists, many people are driving there.
Also, a typical medical office generates three to four times as much traffic as a typical multi-family
complex on the same property, depending on the time of day. We all know this, as whenever we
go to the doctor or dentist, the waiting room is full of people. For medical businesses to be
financially viable, they need several patients per hour, every hour. Also, a typical non-medical
office generates one to two times as much traffic as a typical multi-family complex on the same
property, depending on the time of day. Some offices have many visitors, some do not.
Again, all facts, not opinions.
OK, for a specific proposed development, once the trip generation has been estimated, the trip
distribution is then estimated. Trip distribution estimates how much traffic turns left or right or
travels straight at each of the accesses of the proposed development, and the nearby
intersections. This trip distribution is based on how much of the existing traffic travels to or from
each of the available directions. If the proposed development is located in an undeveloped area,
then the trip distribution is estimated based on future planned land uses and their directions from
the proposed development.
For example, if a new residential community is proposed, then the exiting traffic will be toward
the planned employment and retail centers. Similarly, the entering traffic to a new residential
community, will be determined by the portion from each direction of future planned employment
and retail centers. If the new proposed development is an employment center or a retail center,
then the entering and exiting site traffic will be from and to the future planned residential areas.
Once the various traffic volumes turning left, turning right, and traveling straight at all accesses
and studied street intersections have been estimated, then traffic improvements can be
determined. These improvements might include additional through lanes at nearby intersections,
additional left-turn lanes at nearby intersections, left-turn lanes at accesses, right-turn lanes at
nearby intersections or accesses, or traffic signals at intersections or accesses, or left-turn arrows
at traffic signals at intersections or accesses.
Signals and left-turn arrows are most interesting. Generally, people want traffic signals and leftturn arrows because they think they will have green lights, always. Oddly enough, a green light
for some vehicles is by definition a red light for other people. So, everybody wants a green light
for them and a red light for everyone else. Rather impossible, because each of us is sometimes
one of the everybodies, and other times one of the everyone elses.
Again, facts exist. Everyone one of us gets a red light sometimes and a green light sometimes. A
few decades ago, during a Scottsdale City Council meeting, as Scottsdale Traffic Engineering
Manager, I explained this truth to Mayor Drinkwater and the other six councilmembers. The Mayor
jokingly said to me, “Paul, can’t you time the traffic signals so that everyone always gets a green
light?” I have been asked that same question by many people the past 40 years. My answer is
always the same: by definition, traffic signals are installed to stop cars. If you do not want to stop
cars, then do not construct a traffic signal. Sometimes each of us is the car that stops and
sometimes each of us is the car that goes.
Then the response is something like, “Paul, can’t you time the traffic signals so that most vehicles
get longer greens and shorter reds; and fewer vehicles get shorter greens and longer reds?” Like
most of life, everything has advantages and disadvantages. If only we could eat chocolate all day
and never get cavities or gain weight. Though, yes, we can give longer greens and shorter reds
to more cars. Traffic engineers devote much of their time to counting and observing cars and
trucks to decide which approach has the most vehicles and which approach has the least vehicles.
With these factual data, we can time the signals so that more cars get longer green lights. Rarely,
is it easy to know which approach at what time has the most vehicles. Usually, the approach with
the most vehicles is all four of the approaches.
So, the end result of a Traffic Impact Analysis are recommendations for turn lanes, additional
through lanes, signals, and left-turn arrows at specific locations. Traffic engineers employed by
the appropriate jurisdictions carefully examine the Traffic Impact Analyses to ensure that the
calculations, analyses, discussion, and recommendations conform to scientific data and
methodology. If a public agency traffic engineer disagrees with one or more aspects of a specific
Traffic Impact Analysis, they have the authority to require revisions. These revisions might include
reducing the size of the proposed development – be it number of homes or size of commercial
buildings. They also might require increasing or decreasing the number of driveways, or moving
driveways, or restricting turns at driveways. Sometimes they suggest changing land uses, though
that is typically a zoning issue, not a traffic engineering decision.
These public agency traffic engineers confirm that all aspects of each Traffic Impact Analysis are
accurate and valid, before they approve the Traffic Impact Analysis.
If a proposed development is approved by the jurisdiction decision-makers, the development can be constructed. The recommended traffic improvements also must be constructed. Then the
traffic goes to where the traffic signals and left turn arrows were installed. And, the traffic turns
left and right where the turn lanes were installed. And, the traffic uses the additional travel lanes
that were installed. The recommended improvements become the favored travel routes.
Rather interesting, that all the freeway traffic is exactly where we constructed the freeways.
Development on vacant property or redevelopment on underutilized property is a change. Usually
a change that some people want and other people do not want. The human condition is such that
we want change that is our idea, and oppose change that is not our idea. Each of us has a right
to our opinions – particularly in the United States. Opinions of interested people are essential
considerations for elected representatives. The facts and professional recommendations
contained within Traffic Impact Analyses are vital to the deliberation of decision-makers.
Curious about something traffic?
Call or e-mail Paul at (480) 505-3931 and pbasha@summitlandmgmt.com.
