Sout
h A
fric
an N
atio
nal R
oads
Age
ncy
South African
Highway Capacity Research
TRB WORKSHOP 153SOUTH AFRICAN HIGHWAY TECHNOLOGY AND PRACTICE
Christo van As
Alex van Niekerk
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy
Need for South African Highway Capacity Research?
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy
Need for South African Highway Capacity Research?
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy Need for South African
Highway Capacity Research
�Highway Capacity Manual– Often prescribed as minimum standard for
capacity improvements– Capacity improvement required when
• Level-of-service poorer than • LOS D in 30th Highest hour
�Therefore important to– Validate/Calibrate the HCM– For South African conditions
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy
Recent Contributions
�Capacity Analysis of– Two-lane roads– Multilane highways & Freeways– Signalised intersections
South African National Roads Agency
Capacity R
esearchT
wo-Lane H
ighways
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy Two-Lane Highways
Measures of Effectiveness
�HCM Measures of Effectiveness– Percent Time-Spent-Following (PTSF)– Average Travel Speed
�PTSF, however, difficult to observe– HCM Allows a surrogate measure– Percentage vehicles with headways < 3 s– Termed “Percentage followers”
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy Two-Lane Highways
Research Methodology
�Observations using– Double inductive loops
�Traffic loggers– New traffic loggers capability developed– Directly observes percentage followers– Can be used to directly observe LOS
�Observations on– 25 Different two-lane highways
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy
Two-Lane HighwaysPercentage Followers
�Observations indicate that HCM– Significantly overestimates
Percentage followers
�It was therefore necessary to develop a new local two-lane model
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy Two-Lane Highways
HCM Model: Typical result
South African National Roads AgencyT
wo-Lane H
ighways
New
Model
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy
HCM OverestimationProbably due to shoulder use
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy
Appropriateness ofPercentage Followers
as a
Measure of Effectivenessfor establishing
Level of Service
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy
Percentage followers as aMeasure of Effectiveness
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy
New Measure of Effectiveness“Follower Density”
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy
New Measure of Effectiveness“Follower Density”
�Follower Density– Percentage Followers x Traffic density– Units: Followers per km (per lane)
�Advantages– Gives greater weight to traffic flow– Automatically takes speed into account
South African National Roads Agency
New
SA
NR
AL
Highw
ay Traffic M
odel
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy
HTM Highway Traffic Model
Vertical queue
Platoon (queue) length modelled every 20m over length of road
Macroscopic “simulation”model based on catching-up
and overtaking rates
South African National Roads Agency
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy
Capacity ResearchMultilane Highways & Freeways
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy
Multilane Highways/FreewaysResearch Methodology
�Observations using– Double inductive loops– Newly developed traffic loggers
�Observations on– 20 Different freeways/highways
�Scope of research– Restricted to Basic Freeway Sections– Interchanges excluded
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy
Multilane Highways/FreewaysSpeed/Flow Relationship
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy
Multilane Highways/FreewaysPer Lane Speed/Flow Relationship
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy
Multilane Highways/FreewaysLane Distribution
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy
FreewaysTypical observed capacities
23001900South Africa24002300HCM
Capacity rangePc/hour/lane
Description
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy
Multilane Highways/FreewaysPercentage Followers?
�Multilane Highways/Freeways�Modelled in terms of
– Percentage followers
�Advantage: Uniformity– Follower density used as a MOE for all
types of highways:• Two-lane Highways• Multilane Highways/Freeways
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy
Multilane Highways/FreewaysPercentage Followers/Flow
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy
Multilane Highways/FreewaysSpeed/Percentage Followers
South African National Roads Agency
Signalised Intersections
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy Signalised Intersections
Delay Models
�HCM delay model found to– Overestimate delay significantly
�New model was therefore developed– University research (Pieter Pretorius)– Improve delay estimates significantly
�Study methodology– Simulation evaluations
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy Signalised Intersections
HCM Delay Model
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy Signalised Intersections
New Delay Model
South African National Roads Agency
Conclusions
Conclusions
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy
Conclusions
�Major differences found between USA & South Africa
�A need exists for a South African Highway Capacity Manual
�South Africa has the expertise– That can make a significant contribution– In the field of traffic engineering
Sout
h A
fric
an N
atio
nal R
oads
Age
ncy
Thank You
Images courtesy of SA Tourism
