Speed on Roadway Segments

National Forum on

SpeedingSpeeding

June 15-16, 2005

By Kay Fitzpatrick

Sources of Material for Presentation NCHRP Project 15-18 FHWA Rural Highway Studies TxDOT Research Literature

Topic Areas

Design Speed Posted Speed Operating

Speed

Design Speed

Rural Highway, Horizontal Curves (128 sites, 6 states)

0

20

40

60

80

100

120

140

0 20 40 60 80 100 120 140

Inferred Design Speed (km/h)

85th

Per

cent

ile

Spee

d (k

m/h

)

Suburban Arterial, Horizontal Curves (19 sites, Texas)

01020304050607080

0 20 40 60 80

Inferred Design Speed (mph)

85

th P

erc

en

tile

Sp

ee

d

(mp

h)

Posted Speed

Procedures Used to Set Speed Limits (ITE Survey) 85th percentile speed –

predominant factor Roadway geometry Roadside development Crash experiences Political pressure

Deviations From the 85th Percentile Speed (ITE Survey) Politics (33%) Crashes (13%) Roadway areas (11%) Roadway geometry (9%)

MUTCD Guidelines

85th percentile speed of free-flowing traffic rounded up to nearest 5 mph

Road characteristics Speed pace Roadside development Parking practices Reported crash experiences

85th Speed vs Speed Limit (NCHRP / TxDOT / FHWA Data)

0

20

40

60

80

0 10 20 30 40 50 60 70 80

Posted Speed Limit (mph)

85th

Sp

eed

(m

ph

)

Rural ArterialsS/U ArterialsS/U CollectorsS/U LocalSL = 85th

Percentile Speed Equals Posted Speed (Rural Roads)

Source Functional Class

Vehicle Percentile (%) When Speed =

Sites

Posted + 5 mph + 10 mph

NCHRP Rural, Arterial 37 / 48* 70 91 9

FHWA Rural, Minor Arterial

59 87 99 9

FHWA Rural, Principal Arterial

72 90 98 36

FHWA Rural 64 86 97 126

Percentile Speed Equals Posted Speed (Suburban/Urban Roads)

Source Functional Class

Vehicle Percentile (%) When Speed =

Sites

Posted + 5 mph + 10 mph

TxDOT Suburban/Urban Arterial

32 69 91 35

NCHRP Suburban/Urban Arterial

32 69 92 35

NCHRP Suburban/Urban Collector

23 57 86 22

NCHRP Suburban/Urban Local

52 83 96 13

Operating and Posted SpeedField Studies Previous findings:

85th %-ile speed exceeds posted 50th %-ile speed near posted

NCHRP/other studies:Rural: 37 to 72% at posted speedSuburban/urban: 32 to 52% at

posted

Speed Zoning Reports

ITE TENC Committee 97-12 Request “speed zoning

investigations your agency has recently conducted”

256 report received 128 contained both 85th percentile

speed value and speed limit recommendation

Operating and Posted SpeedSpeed Zone Studies

128 speed zone studies 10% rounded up and 31% rounded to nearest 5 mph

0

25

50

75

100

-5 0 5 10 15 20

Cu

m F

req

Dif (85th – Recom SL)

Operating and Posted SpeedComments 85th percentile speed is a “starting

point” Encourage changes in how speed

limits are set?

Operating Speed

FHWA StudyRural Highways

Directionof Travel

Piezoelectric Sensor

Traffic Counter/Classifier

85t

h %

-ile

Spe

ed (

km/h

)

50

70

90

110

0 250 500 750 1000Radius (m)

> 4%0 to 4 %0 to -4%< - 4%

FHWA Study - Rural HighwaysSpeeds on Horizontal Curves

K-Value (m/%)

85th

%-il

e S

peed

(km

/h)

50

70

90

110

130

0 20 40 60

LSD- FHWA

LSD- NCHRP

FHWA Study - Rural HighwaysLimited Sight Distance Curves

FHWA Study - Rural HighwaysFindings Horizontal Alignment

Radius: key variable Other variables: length of curve,

deflection angle, superelevation, desired speed (previous tangent speed)

Vertical Alignment K, SSD

Combination Alignments Theories

NCHRP Field Studies

79 tangent sites, most suburban/urban

Sites not near signals or horizontal curves (elements known to influence operating speed)

Free-flow speed measured with laser and counters

Roadway and roadside characteristics

0

15

30

45

60

0 15 30 45 60

Posted Speed Limit (mph)

85th %

-lie

Sp

eed

(m

ph

)

LocalCollectorsArterial, C&G

Arterial, ShoulderSL = 85th

NCHRP StudyPosted Speed Limit

85th %-ile Speed (mph)

Cu

m D

ist

(%)

0

20

40

60

80

100

0 10 20 30 40 50 60 70

Arterials, 69 sites

Collectors, 20 sites

Locals, 13 sites

NCHRP StudyRoadway Type

0

15

30

45

60

75

0 30 60 90 120 150 180

Access Density (pts/mi)

85th

%-i

le S

pee

d (

mp

h) Local

CollectorsArterial, C&GArterial, Shoulders

NCHRP StudyAccess Density

NCHRP StudyParking

0

10

20

30

40

50

60

70

1 2

Parking (1=yes, 2=no)

85th

Per

cen

tile

Sp

eed

(m

ph

)

0

10

20

30

40

50

60

70

1 2

Parking (1=yes, 2=no)

85th

Per

cen

tile

Sp

eed

(m

ph

)

LocalCollectorsArterial, C&GArterial, Shoulder

LocalCollectorsArterial, C&GArterial, Shoulder

NCHRP StudyPedestrian Activity

0

10

20

30

40

50

60

70

1 2 3

Pedestrian Activity (1=low, 2=med, 3=high)

85th

Per

cen

tile

Sp

eed

(m

ph

)

0

10

20

30

40

50

60

70

1 2 3

Pedestrian Activity (1=low, 2=med, 3=high)

85th

Per

cen

tile

Sp

eed

(m

ph

)

LocalCollectorsArterial, C&GArterial, Shoulder

LocalCollectorsArterial, C&GArterial, Shoulder

NCHRP StudyCenterline Markings

0

10

20

30

40

50

60

70

1 2

Centerline Markings (1=yes, 2=no)85th

Per

cen

tile

Sp

eed

(m

ph

)

0

10

20

30

40

50

60

70

1 2

Centerline Markings (1=yes, 2=no)85th

Per

cen

tile

Sp

eed

(m

ph

)LocalCollectorsArterial, C&GArterial, Shoulder

LocalCollectorsArterial, C&GArterial, Shoulder

NCHRP StudyMedian Width

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70 80 90

Median Width (ft)85th

Per

cen

tile

Sp

eed

(m

ph

)

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70 80 90

Median Width (ft)85th

Per

cen

tile

Sp

eed

(m

ph

)

LocalCollectorsArterial, C&GArterial, Shoulder

LocalCollectorsArterial, C&GArterial, Shoulder

NCHRP StudyTotal Pavement Width

0

10

20

30

40

50

60

70

0 20 40 60 80 100 120 140

Total Pavement Width (ft)85th

Per

cen

tile

Sp

eed

(m

ph

)

0

10

20

30

40

50

60

70

0 20 40 60 80 100 120 140

Total Pavement Width (ft)85th

Per

cen

tile

Sp

eed

(m

ph

)

LocalCollectorsArterial, C&GArterial, Shoulder

LocalCollectorsArterial, C&GArterial, Shoulder

Several variables show influences: Access density Pedestrian activity Absence of centerline or edge line

markings On-street parking Median presence

Additional data needed

NCHRP StudyInfluence on Operating Speed

Summary

Influence on Horizontal Curve Speed

Pre 1990

sRural

Krammes

1993Rural

Fitz1999Rural

Schurr

2002Rural

Poe1996Low

Fitz2000Urba

n

DC / Radius

Y Y Y Y

Deflect Y Y Y

Len Curve Y Y

Inferred Sp

Y

Lane Width

Y

Access Y Y

Speed Limit

Y Y

Grade Y Y Y

Influence on Tangent SpeedParm

aRural1999

Dixon

Rural

1999

PolusRural2000

Fitz TX Urban2000

Fitz TXUrban2000

FitzUrban2003

Lane Width Y

Nearby HC Y O

Access SL O SL Y

Speed Limit Y Not Y

Grade Y O SL SL

Pedestrian O

Median O

Parking O

SL=study limited, O = may, Y = yes

Questions?