1

Roadway Functional

Classification

MnDOT

Traffic Topics

July 16, 2014

Howard Preston, PE

2

Agenda

1. Overview & Basics

2. System Components

3. System Characteristics

4. Design Considerations

5. Conclusions

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Functional

Classification Overview

One of the basic concepts in transportation planning.

Based on the two primary functions of roadways: Land Access

Mobility

The primary components of a functions system include: Principal Arterials

Minor Arterials

Collectors

Local Streets/Roads

Why do a functional system? Consistent with AASHTO geometric design policies

Compatibility with land planning

Financial – couldn’t afford a system made up entirely of principal arterials

Roadways that only serve one function are safer and more efficient

Balance is best

System Component Characteristics

4

AASHTO’s Green Book

5

AASHTO’s

Green Book

–

Table of

Contents

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AASHTO’s

Green Book – Forward

…”Most of the technical material that follows is

detailed or descriptive design information.

Design Guidelines are included for freeways,

arterials, collectors, and local roads, in both

urban and rural location, paralleling the

functional classification used in highway

planning. The book is organized into functional

chapters to stress the relationship between

highway design and highway function. An

explanation of functional classification is included

in Chapter 1.”

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Roadway Functional

Classification

System Components

8

9

Rural Miles by Type of Rural Road

Functional Classification (U.S. Data)

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Roadway Functional

Classification

System Characteristics

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12

Minnesota’s Road System

Roadway

Class

Functional

ClassMiles MVMT

Crashes

(Rates)

Fatalities

(Rates)

Interstate Principle Arterial914

(0.7%)

11,431

(22%)

12,574

(1.1)

50

(0.4)

Trunk HighwayPrincipal Arterial

& Minor Arterial

11,022

(8%)

19,349

(37%)

29,023

(1.5)

269

(1.4)

CountyMinor Arterial &

Collector

45,430

(33%)

12,748

(25%)

29,320

(2.3)

226

(1.8)

CityMinor Arterial,

Collector & Local

18,460

(14%)

7,321

(14%)

29,284

(4.0)

49

(0.7)

Other Local59,531

(44%)

1,094

(2%)

3,390

(3.1)

31

(2.9)

State Totals 135,357 51,943 103,591 625

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Crash Rate by Roadway Type

14

Access vs. Crash Rate

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0%

5%

10%

15%

20%

25%

30%

35%

% C

rash

es/L

en

gth

Access Density

Total Crash % Length % Total Crash Density

State Trunk Highway System County Highway System

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Comparison of Capacities

by Functional Classification

Freeway Expressway Urban/

Suburban

Arterial

Local

Streets

Principal Arterial Minor Arterial

2,400

1,200

800

? Capacity is rarely

an issue on these

types of roads

1,000

2,000

3,000

Capacity (

veh/h

r/la

ne)

600

Urban/

Suburban

Collector

Collector Local

This is not suggesting that the classification of

the roadway affects traffic operations – it

illustrates how the differing characteristics

(signal spacing, level of access, pedestrians,

transit, parking, etc.) associated with the

classifications do affect operations.

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Spacing Guidelines for

Functionally Classified Roads

Land Use Characteristics

Principal Arterials Minor Arterials Collectors Local Streets

Developed Areas 2 to 3 Miles 1/4 to 1/2 Mile 1/8 to 1/2 Mile As Needed to

Access Land Uses

Developing Areas 3 to 6 Miles 1 to 2 Miles 1/2 to 1 Mile As Needed to

Access Land Uses

Rural Areas 6 to 12 Miles 4+ Miles As Needed to

Access Land Uses As Needed to

Access Land Uses

Source: Metropolitan Council, Metropolitan Development Guide, Appendix F and Federal Highway Administration, Highway Functional Classification

• A real challenge is urbanizing areas is to provide a road system that matches the density of

the planned development.

• Urban densities of development need urban densities of roadways.

• There are consequences for under building road systems – higher volumes on the existing

parts of the system.

• Timing the expansion of the road system is an important consideration – when does the

window of opportunity open?

• Need a more dense

road system in urban

areas to handle higher

traffic volumes.

• Rural system is based

on the 1 mile section

road concept.

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Schematic Illustration of a Portion of

a Suburban Street Network

Source: AASHTO Green Book

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Existing Functional Class - Urban

Classification Miles %

Principal 69 7%

Arterial (A&B) 189 18%

Collector 147 14%

Local 648 61%

TOTAL 1053

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Existing Functional Class - Rural

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Roadway Functional

Classification

Design Considerations

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Balance

Safety

Design Considerations

Aesthetics

Community Goals

Environmental Issues

Designers are encouraged to find the “Appropriate”

balance between design consistency, safety,

aesthetics, environmental issues and community

goals.

How do you find this point?

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Basic Design Feature – Lane Width

1. A Policy on Geometric Design of Streets and Highways

2. Highway Functional Classification Concepts, Criteria and Procedures

3. Road Design Manual

Source Basic By Functional Class

Arterial Collector Local

AASHTO1 9 ft – 12 ft 11 ft – 12 ft 10 ft – 12 ft 9 ft – 12 ft

FHWA2 8 ft – 12 ft 11 ft – 12 ft

10 ft – 12 ft

8 ft – 12 ft

MnDOT3 12 ft ? ? ?

Does include reference to AASHTO

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Issues – Mobility vs. Access

Strategy for Prioritization

MAJOR ARTERIALS are primarily intended to move traffic and

access to these roadways should be highly managed to optimize

efficiency and safety.

LOCAL STREETS are primarily intended to provide access to

abutting property and these roadways should be designed to

minimize speeds, volumes, and through traffic with only minor

access-related restrictions.

MINOR ARTERIALS and COLLECTORS have to serve the dual

functions of moving traffic and land access. A moderate level of

access management, including features such as turning lanes,

medians, minimum driveway separations is appropriate to mitigate

the adverse affects associated with closely spaced driveways and

high levels of turning traffic.

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Mn/DOT Access Spacing Guidelines

Source: Mn/DOT’s Access Manual

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Dakota County Access Spacing

Guidelines

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Recommended Rural

Intersection Design Policy

Source: Bypass Lane Safety, Operations and Design Study. Mn/DOT, 2000-22

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28

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Recommended Rural

Intersection Street Lighting Policy

Priority

Major Street Functional Classification

(Major Street Volumes in Vehicles Per Day)

Principal

Arterial

Minor Arterial

(TH or

CSAH)

Collector

(CSAH or

CR)

Local

(CR or Twp.

Rd.)

Low 0 – 2,000

(10%)

0 – 1,000

(10%)

0 – 500

(10%)

0 – 250

(10%)

Moderate 2,000 – 5,000

(15%)

1,000 – 2,000

(15%)

500 – 1,000

(15%)

250 – 500

(15%)

High > 5,000

(20%)

> 2,000

(20%)

> 1,000

(20%)

> 500

(20%)

NOTE: The value in parentheses is the percent of the major street traffic that is recommended

on the minor crossing street for the warrants/guidelines to be met.

Source: Safety Impacts of Street Lighting at Isolated Intersections. Mn/DOT, 1999-17

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Complete Streets Initiative

MnDOT Policy on Complete Streets does NOT

mention Functional Classification.

MnDOT Policy does recognize that “Complete Streets

does not mean all modes on all roads.”

This guidance appears to suggest that different

outcomes for different roads is expected and that

modal planning efforts should designate priority

corridors.

Consideration of Functional Classification could help

with this corridor prioritization effort.

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Conclusions

Roadways have 2 basic functions – mobility and access.

Consideration of the functionally classified highway system is an important part of understanding issues related to:

Land planning

Highway corridor evaluation

Safety and traffic operations studies

Geometric design features

Context sensitive design

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Conclusions

There are consequences for ignoring the

concept of functional systems.

Most traffic studies need to address

“actual” vs. “expected” conditions. The

characteristics associated with the

components of a functionally classified

system provide important insight as to what

is expected.

A balance is best.