1.3.4 characteristics of classifications...geometric design guide for canadian roads january 2002...

Geometric Design Guide for Canadian Roads

.3.4.1January 2002

1.3.4 CHARACTERISTICSOF CLASSIFICATIONS

The principal characteristics of each of the sixgroups of road classifications are described by

Figure 1.3.4.1 Relationship of Urban Road Classifications

the following figure and tables. Figure 1.3.4.1illustrates the desirable interrelationship of theurban road classification groups. Tables 1.3.4.1and 1.3.4.2 provide summaries of the typicalcharacteristics of the various groups and sub-groups, for rural and urban roads respectively.

expressway

freeway

expressway

major arterial

minor arterial

collector

local

public lane

signalized intersection

cul-de-sac

expressway

major arterial

major arterial

minorarterial

freeway

local

lane

123456123456123456123456collector

Design Classification

Page 1.3.4.2 January 2002

Rural Locals Rural Collectors Rural Arterials Rural Freeways

service function traffic movement traffic movement traffic movement optimum mobilitysecondary and land access primary

consideration of equal considerationimportance

land service land access traffic movement land access no accessprimary and land access secondary

consideration of equal considerationimportance

traffic volumevehicles per day <1000 AADT <5000 AADT <12 000 AADT >8000 AADT(typically)

flow interrupted flow interrupted flow uninterrupted freeflow (gradecharacteristics flow except at separated)

majorintersections

design speed(km/h) 50 - 110 60 - 110 80 - 130 100 - 130

average runningspeed (km/h)(free flow 50 - 90 50 - 90 60 - 100 70 - 110conditions)

vehicle type predominantly all types, up to all types, up to all types, up topassenger cars, 30% trucks in the 20% trucks 20% heavylight to medium 3 t to 5 t range trucks

trucks andoccasional heavy

trucks

normal locals locals collectors arterialsconnections collectors collectors arterials freeways

arterials freeways

Table 1.3.4.1 Characteristics of Rural Roads


1.4.3 OPERATING SPEEDCONSISTENCY

The safety of a road is closely linked tovariations in the speed of vehicles travelling onit. These variations are of two kinds:

1. Individual drivers vary their operatingspeeds to adjust to features encounteredalong the road, such as intersections,accesses and curves in the alignment. Thegreater and more frequent are the speedvariations, the higher is the probability ofcollision.

2. Drivers travelling substantially slower orfaster than the average traffic speed havea higher risk of being involved in collisions.

A designer can therefore enhance the safety ofa road by producing a design which encouragesoperating speed uniformity.

As noted in the discussion of speed profiles inChapter 1.2, simple application of the designspeed concept does not prevent inconsistenciesin geometric design. Traditional North Americandesign methods have merely ensured that alldesign components meet or exceed a minimumstandard, but have not necessarily ensuredoperating speed consistency betweencomponents.

Practices used in Europe and Australia havesupplemented the design speed concept withmethods of identifying and quantifyinggeometric inconsistencies in horizontalalignments of rural two-lane highways. This typeof road typically has the most problems relatedto design inconsistency. These methods havenot been perfected, particularly in predicting theperformance of a newly designed road. Theireffectiveness is greater in evaluating existingroads and identifying priority improvements toreduce collision rates.

1.4.3.1 Prediction of OperatingSpeeds

In order to establish consistency of horizontalalignment design for a proposed new road, it isnecessary to predict operating speeds

January 2002

associated with different geometric elements.Limited information is generally available toassist designers with prediction of operatingspeeds, but the material presented in thissection may help, noting the limited databaseused. As an alternative, some jurisdictions havelocal data on which to base speed predictions.

Researchers9 collected data from five US states,measuring 85th percentile operating speeds,under free flowing traffic conditions, on longtangents and horizontal curves on rural two-lanehighways. Long tangents (250 m or more) arethose lengths of straight road on which a driverhas time to accelerate to desired speed beforeapproaching the next curve. The mean85th percentile speed on long tangents wasfound to be 99.8 km/h on level terrain and96.6 km/h in rolling terrain. It was noted thatthese speeds were probably constrained by the90 km/h posted speed in force at the time.

On horizontal curves, the research foundconsistent disparities between 85th percentilespeeds and inferred design speeds, with thegreater disparity on tighter radius curves. The85th percentile speed exceeded the designspeed on a majority of curves in each 10 km/hincrement of design speed up to a design speedof 100 km/h. At higher design speeds, the85th percentile speed was lower than the designspeed.

Using regression techniques, a relationship wasfound between 85th percentile speeds and thecharacteristics of a horizontal curve.

V85 = 102.45 + 0.0037L (1.4.1) – (8995 + 5.73L) / R

Where V85 = 85th percentilespeed on curve (km/h)

L = length of curve (m)

R = radius of curvature (m)

1.4.3.2 Speed Profile Model

The findings outlined in Subsection 1.4.3.1support the conclusion that there is no strongrelationship between design speed andoperating speed on horizontal curves.

Page 1.4.3.1

Design Consistency

Consistency of horizontal alignment designcannot therefore be assured by using designspeed alone. A further check can be carried outby constructing a speed profile model, usingpredicted 85th percentile operating speeds fornew road and measured 85th percentile speedsfor existing roads.

For the predictive model, it is necessary tocalculate the critical tangent length betweencurves as follows:

(1.4.2)

Where: TLC

= critical tangent length (m)

Vf

= 85th percentile desiredspeed on long tangents(km/h)

V85n

= 85th percentile speed oncurve n (km/h)

a = acceleration/decelerationrate, assumed to be0.85 m/s2

The calculation assumes that decelerationbegins where required, even if the beginning ofthe curve is not yet visible.

Each tangent is then classified as one of threecases, as shown on Figure 1.4.3.1, bycomparing the actual tangent length (TL) to thecritical tangent length (TL

C).

Having found the relationship between eachtangent length (TL) and the critical tangentlength (TL

C), the equations in Table 1.4.3.1 can

then be used, as appropriate, to construct thespeed profile model.

a92.2585V85VV2

TL2

22

12

fc

−−=

Page 1.4.3.2

The speed profile model is used to estimate thereductions in 85th percentile operating speedsfrom approach tangents to horizontal curves, orbetween curves. Designers should note that theresearch6,9 on which the model is based dealtonly with two-lane rural highways. The sameprinciples, however, can be applied to design ofother classes of roads.

1.4.3.3 Safety

As noted in Chapter 1.2, there is evidence thatthe risk of a collision is lowest near the averagespeed of traffic and increases for vehiclestravelling much faster or slower than average.While this is true in relation to the generaldistribution of speeds in a stream of traffic, ithas also been found to apply when there is avariation in speeds caused by the effects of areduction in speed from one geometric elementto the next.

This particular form of speed variation may beexperienced in situations such as the transitionfrom a tangent to a curve, or between curves.In these cases, the previously mentionedresearch study9, found that the mean collisionrate increased in direct proportion to the meanspeed difference caused by the transition fromone geometric element to the other. The resultsare noted in Figure 1.4.3.2.

The numerical values from Figure 1.4.3.2should be used with caution, because of thedatabase used. Wherever possible, designersshould use local data. However, the principlesestablished show the effect of a lack ofhorizontal alignment consistency on increasingcollision potential. Figure 1.4.3.2 should not beinterpreted to mean that collision rate decreasesas speed decreases.

September 1999


Page 2.1.2.13January 2002

Des

ign

Sp

eed

(km

/h)

4050

6070

8090

100

110

120

130

AA

AA

AA

AA

AA

Rad

ius

e2

3&4

e2

3&4

e2

3&4

e2

3&4

e2

3&4

e2

3&4

e2

3&4

e2

3&4

e2

3&4

e2

3&4

(m)

lane

lane

lane

lane

lane

lane

lane

lane

lane

lane

lane

lane

lane

lane

lane

lane

lane

lane

lane

lane

7000

NC

NC

NC

NC

NC

NC

NC

NC

RC

RC

710

710

5000

NC

NC

NC

NC

NC

NC

NC

RC

555

555

RC

580

580

RC

600

600

4000

NC

NC

NC

NC

NC

NC

RC

475

475

RC

495

495

RC

515

515

0.02

354

054

030

00N

CN

CN

CN

CN

CR

C39

040

0R

C41

041

00.

020

430

430

0.02

445

045

00.

036

465

465

2000

NC

NC

NC

RC

275

275

RC

300

300

0.02

330

035

00.

022

335

335

0.02

935

035

00.

034

365

365

0.04

038

038

015

00N

CN

CR

C22

522

5R

C25

025

00.

024

250

250

0.02

927

027

50.

029

290

290

0.03

630

530

50.

042

315

315

0.04

933

033

512

00N

CN

CR

C20

020

00.

023

225

225

0.02

822

522

50.

033

240

240

0.03

426

026

00.

043

270

270

0.04

928

529

00.

055

295

320

1000

NC

RC

170

170

0.02

117

517

50.

027

200

200

0.03

220

020

00.

037

225

225

0.04

023

523

50.

048

245

255

0.05

426

028

00.

058

280

300

900

NC

RC

150

150

0.02

317

517

50.

029

180

180

0.03

420

020

00.

039

200

200

0.04

322

522

50.

051

235

250

0.05

725

027

00.

060

280

300

800

NC

RC

150

150

0.02

516

016

00.

031

175

175

0.03

617

517

50.

042

200

200

0.04

621

021

50.

054

220

240

0.05

925

026

0m

inR

=95

070

0N

C0.

021

140

140

0.02

715

015

00.

034

175

175

0.03

917

517

50.

045

185

195

0.05

020

021

00.

058

220

235

0.06

025

026

060

0N

C12

012

00.

024

125

125

0.03

014

014

00.

037

150

150

0.04

217

517

50.

048

175

185

0.05

419

020

00.

060

220

220

min

R=

750

500

RC

100

100

0.02

712

012

00.

034

125

125

0.04

114

015

00.

046

150

160

0.05

216

017

50.

059

190

190

0.06

022

022

040

00.

023

9090

0.03

110

010

00.

038

115

120

0.04

512

513

50.

051

135

150

0.05

716

016

50.

060

190

190

min

R=

600

350

0.02

590

900.

034

100

100

0.04

111

011

50.

048

120

125

0.05

412

514

00.

059

160

160

min

R=

440

300

0.02

880

800.

037

9010

00.

044

100

110

0.05

112

012

50.

057

125

135

0.06

016

016

025

00.

031

7580

0.04

085

900.

048

9010

00.

055

110

120

0.06

012

512

5m

inR

=34

022

00.

034

7080

0.04

380

900.

050

9010

00.

057

110

110

0.06

012

512

520

00.

036

7075

0.04

575

900.

052

8510

00.

059

110

110

min

R=

250

180

0.03

860

750.

047

7090

0.05

485

900.

060

110

110

160

0.04

060

750.

049

7085

0.05

685

90m

inR

=19

014

00.

043

6070

0.05

265

800.

059

8590

Not

es:

120

0.04

660

650.

055

6575

0.06

0•

eis

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rele

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n10

00.

049

5065

0.05

865

70m

inR

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0•

Ais

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met

res

900.

051

5060

0.06

065

70•

NC

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0.05

450

600.

060

6570

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Cis

rem

ove

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rse

crow

nan

dsu

pere

leva

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norm

alra

te70

0.05

650

60m

inR

=90

•S

pira

llen

gth,

L=

A2

/Rad

ius

600.

059

5060

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pira

lpar

amet

ers

are

min

imum

and

high

erva

lues

may

beus

ed0.

059

5060

•F

or6

lane

pave

men

t:ab

ove

the

dash

edlin

eus

e4

lane

valu

es,

min

R=

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belo

wth

eda

shed

line,

use

4la

neva

lues

x1.

15.

e max

=0.

06•

Adi

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dro

adha

ving

am

edia

nle

ssth

an3.

0m

wid

em

aybe

trea

ted

asa

sing

lepa

vem

ent.

0.0

30

0.0

28

Tab

le 2

.1.2

.6S

up

erel

evat

ion

an

d M

inim

um

Sp

iral

Par

amet

ers,

em

ax =

0.0

6 m

/m1

Alignment and Lane Configuration


Des

ign

(Sp

eed

km/h

)40

5060

7080

9010

011

012

013

0

AA

AA

AA

AA

AA

Rad

ius

e2

3&4

e2

3&4

e2

3&4

e2

3&4

e2

3&4

e2

3&4

e2

3&4

e2

3&4

e2

3&4

e2

3&4

(m)

lane

lane

lane

lane

lane

lane

lane

lane

lane

lane

lane

lane

lane

lane

lane

lane

lane

lane

lane

lane

7000

NC

NC

NC

NC

NC

NC

NC

RC

RC

RC

710

710

5000

NC

NC

NC

NC

NC

NC

RC

RC

555

555

RC

580

580

0.02

160

060

040

00N

CN

CN

CN

CN

CN

CR

C48

048

0R

C49

549

50.

021

515

515

0.02

654

054

030

00N

CN

CN

CN

CN

CR

C39

040

0R

C41

041

00.

023

430

430

0.02

845

045

00.

033

465

465

2000

NC

NC

NC

RC

270

275

0.02

130

030

00.

026

300

300

0.02

633

533

50.

033

350

350

0.04

036

536

50.

047

380

380

1500

NC

NC

RC

225

225

0.02

125

526

00.

027

250

250

0.03

227

027

50.

033

290

290

0.04

230

530

50.

050

315

330

0.05

833

036

512

00N

CN

CR

C20

020

00.

026

220

225

0.03

222

522

50.

038

240

240

0.04

026

026

00.

050

270

285

0.05

928

532

00.

067

295

350

1000

NC

RC

170

170

0.02

317

517

50.

029

200

200

0.03

620

020

00.

043

225

225

0.04

724

024

00.

057

250

280

0.06

626

031

00.

074

280

340

900

NC

RC

150

150

0.02

517

517

50.

032

180

180

0.03

920

020

00.

046

200

220

0.05

122

524

00.

062

235

275

0.07

125

030

00.

078

280

330

800

NC

0.02

015

015

00.

027

160

160

0.03

517

517

50.

042

175

195

0.04

920

022

00.

055

210

235

0.06

622

027

00.

075

250

295

0.08

028

033

070

0N

C0.

023

140

140

0.03

015

015

00.

038

165

165

0.04

617

519

00.

053

185

200

0.06

120

023

00.

072

220

260

0.07

925

028

50.

080

280

300

600

RC

120

120

0.02

612

512

50.

034

140

140

0.04

215

016

00.

050

165

185

0.05

817

520

00.

067

190

225

0.77

220

250

0.08

025

028

5m

inR

=83

050

00.

021

100

100

0.03

012

012

00.

039

125

135

0.04

814

015

00.

056

150

175

0.06

416

020

00.

073

190

215

0.80

220

250

min

R=

670

400

0.02

590

900.

035

100

110

0.04

511

512

50.

054

125

150

0.06

313

516

50.

071

160

185

0.08

019

020

0m

inR

=53

035

00.

028

9090

0.03

810

010

50.

049

110

125

0.05

812

015

00.

067

125

160

0.07

516

017

50.

080

190

200

300

0.03

180

900.

042

9010

00.

053

100

120

0.06

312

014

00.

072

125

150

0.08

016

017

5m

inR

=39

025

00.

035

7585

0.04

785

100

0.05

910

012

00.

069

110

135

0.07

812

515

00.

080

160

175

220

0.03

970

800.

051

8010

00.

062

9511

00.

073

110

125

0.08

012

515

0m

inR

=30

020

00.

041

7080

0.05

480

100

0.06

590

110

0.07

511

012

5m

inR

=23

018

00.

044

6580

0.05

775

950.

068

9010

50.

078

110

120

160

0.04

765

800.

060

7590

0.07

285

100

0.08

011

012

014

00.

051

6575

0.06

470

900.

076

8510

0m

inR

=17

0N

otes

:12

00.

055

6075

0.06

970

850.

080

8595

•e

issu

pere

leva

tion

100

0.06

155

700.

074

6580

0.08

085

95•

Ais

spira

lpar

amet

erin

met

res

900.

064

5570

0.07

765

80m

inR

=12

0•

NC

isno

rmal

cros

sse

ctio

n80

0.06

755

650.

080

6575

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Cis

rem

ove

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rse

crow

nan

dsu

pere

leva

teat

norm

alra

te70

0.07

150

600.

080

6575

•S

pira

llen

gth,

L=

A2

/Rad

ius

600.

075

5060

min

R=

80•

Spi

ralp

aram

eter

sar

em

inim

uman

dhi

gher

valu

esm

aybe

used

500.

080

5060

•F

or6

lane

pave

men

t:ab

ove

the

dash

edlin

eus

e4

lane

valu

es,

0.08

050

60•

belo

wth

eda

shed

line,

use

4la

neva

lues

x1.

15.

min

R=

50

max

•A

divi

ded

road

havi

nga

med

ian

less

than

3.0

mw

ide

may

betr

eate

das

asi

ngle

pave

men

t.

0.03

00.

035

Tab

le 2

.1.2

.7S

up

erel

evat

ion

an

d M

inim

um

Sp

iral

Par

amet

ers,

em

ax=

0.08

m/m

1



Figure 2.1.2.12 Lateral Clearance for Range of Lower Values of StoppingSight Distance2

0 1 2 3 4 5 6 7 8 9 10


Page 2.1.2.42 September 1999

Figure 2.1.2.13 Lateral Clearance for Passing Sight Distance2



Figure 2.1.8.3 Performance Curves for Heavy Trucks, 180 g/W, Decelerations& Accelerations7



Figure 2.1.8.4 Performance Curves for Heavy Trucks, 200 g/W, Decelerations& Accelerations7


Page 2.3.1.11September 1999

and limitations. When a design is incompatiblewith the attributes of a driver, the chances fordriver error increases. Inefficient operation andcollisions are often a result.3

In general, traffic volume is the most significantcontributor to intersection collisions. Typically,as traffic volumes increase, conflicts increase,and therefore the number of collisions increase.5

Severity of collisions varies only slightly amongrural, suburban and urban intersections; thepercent of severe collisions is approximately 5%higher for rural intersections.5

Other elements related to intersection collisionrates include geometric layout and trafficcontrol.3 As previously noted, traffic controlmeasures are not addressed in this document.The relationship of specific geometric elementsand safety is described below:

Type of Intersection

In rural settings, four-legged intersectionstypically have higher collision rates thanT-intersections (three-legged) for stop andsignal controls.7

In urban settings, very little difference in collisionrates between four-legged and T-intersectionswas found for low volume intersections(Average Daily Traffic under 20 000); however,for larger volumes, the four-legged intersectionwas found to have the higher collision rate.8

Sight Distance

In both an urban and a rural setting,studies have shown that the collision rate atmost intersections will generally decrease whensight obstructions are removed, and sightdistance increased.3

Channelization

In a rural environment, it was found that left-turn lanes would reduce the potential of passingcollisions.9

In an urban setting, it was found that multi-vehicle collisions decrease when lane “dividers”(raised reflectors, painted lines, barriers or

medians) are used; however the use of left-turnlanes was not considered effective as a collisioncountermeasure but was considered effective asa means of increasing capacity.8

Cross Section

Safety considerations for cross sectionelements, such as lane width, are addressedin Chapter 2.2.

2.3.1.7 Intersection SpacingConsiderations

Both rural road and urban road network spacingis often predicated on the location of the originalroad allowances prior to urban development.The systems of survey employed in the layoutof original road allowances vary from region toregion across Canada. As rural areas urbanize,the development of major roads generallyoccurs along these original road allowances,and consequently road networks vary fromregion to region. As examples, the land surveysystem in Ontario has created a basic spacingbetween major roads of 2.0 km, whereas theland survey system in the prairie provinces hasresulted in a 1.6 km grid.

As development occurs, this spacing is oftenreduced. In areas of commercial or mixed usedevelopment, the traffic generated byemployment and retail shopping may result ina reduced arterial spacing. In downtown areas,this spacing could be reduced further asdetermined by the traffic needs and thecharacteristics of the road network.

The spacing of intersections along a road in bothan urban and rural setting has a large impact onthe operation, level of service, and capacity ofthe roadway. Ideally, intersection spacing alonga road should be selected based on function,traffic volume and other considerations so thatroads with the highest function will have the leastnumber (greatest spacing) of intersections (therelationship of road classification and thepreferred functional hierarchy of circulation isdescribed in Chapter 1.3 of this Guide). However,it is often not always possible to provide idealintersection spacing, especially in an urban

Intersections


setting. As such, the following should beconsidered:

Arterials

Along signalized arterial roads, it is desirable toprovide spacing between signalized intersectionsconsistent with the desired traffic progressionspeed and signal cycle lengths. By spacing theintersections uniformly based on known orassumed running speeds and appropriate cyclelengths, signal progression in both directions canbe achieved. Progression allows platoons ofvehicles to travel through successiveintersections without stopping. For a progressionspeed of about 50 km/h and a cycle length of60 s, the corresponding desired spacing betweensignalized intersections is approximately 400 m.As speeds increase the optimal intersectionspacing increases proportionately. Furtherinformation on the spacing of signalizedintersections is provided in theSubsection 2.3.1.8.

A typical minimum intersection spacing alongarterial roadways is 200 m, generally onlyapplicable in areas of intense existingdevelopment or restrictive physical controlswhere feasible alternatives do not exist. The200 m spacing allows for minimum lengths ofback to back storage for left turning vehicles atthe adjacent intersections.

The close spacing does not permit signalprogression and therefore, it is normallypreferable not to signalize the intersection thatinterferes with progression along a majorarterial. Intersection spacing at or near the200 m minimum is normally only acceptablealong minor arterials, where optimizing trafficmobility is not as important as along majorarterials.

Where intersection spacing along an arterialdoes not permit an adequate level of trafficservice, a number of alternatives can beconsidered to improve traffic flow. Theseinclude: conversion from two-way to one-wayoperation, the implementation of culs-de-sac forminor connecting roads, and the introductionof channelization to restrict turning movementsat selected intersections to right turns only.

On divided arterial roads, a right-in, right-outintersection without a median opening may bepermitted at a minimum distance of 100 m froman adjacent all-directional intersection. Thedistance is measured between the closestedges of pavement of the adjacent intersectingroads.

In retrofit situations, the desired spacing ofintersections along an arterial is sometimescompromised in consideration of other designcontrols, such as, the nature of existing adjacentdevelopment and the associated access needs.

Collectors

The typical minimum spacing between adjacentintersections along a collector road is 60 m.

Locals

Along local roads, the minimum spacingbetween four-legged intersections is normally60 m. Where the adjacent intersections arethree-legged a minimum spacing of 40 m isacceptable.

Cross Roadway Intersection Spacing Adjacentto Interchanges

The upper half of Figure 2.3.1.6 indicates theintersection spacing along an arterial crossingroad approaching a diamond interchange. Thesuggested minimum distance between acollector road and the nearest ramp, asmeasured along the arterial cross road, is200 m (dimension A

c on Figure 2.3.1.6). In the

case of an arterial/arterial cross roadintersection, this minimum offset distance fromthe ramp is normally increased to 400 m(dimension A

a on Figure 2.3.1.6). The same

dimensions apply to arterial cross roadsapproaching parclo-type interchanges as shownon the lower half of Figure 2.3.1.6.

Ramp Intersection Spacing at Interchanges

The upper half of Figure 2.3.1.6, as well asFigure 2.3.1.7, illustrate the suggested andminimum intersection spacing and laneconfigurations on the cross road at a typicaldiamond interchange. The two differentchannelization treatments illustrate the following



Figure 2.3.8.6 Left-Turn Lane Designs Along Four-Lane UndividedRoadways, No Median

Intersections

Page 2.3.8.10 September 1999

Figure 2.3.8.7 Introduced Raised Median

1.3.4 characteristics of classifications...geometric design guide for canadian roads january 2002...

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