E-4-2 Traffic Appraisal Manual, Chapter 13

8/18/2019 E-4-2 Traffic Appraisal Manual, Chapter 13

1/26

CHAEYCER 3 : OPEFULTIONAL APPRAISAL

13.1 GENERAL

13.2 EXAMINING THE OPERATIONAL FEATURES OF A SCHEME

13.3 THE TOOLS OF OPERATIONAL APPRAISAL

13.4 THE USE OF CORDON ISOLATION TO EXAMINE

CONGESTED NETWORKS

13.5 JUNCTION APPRAISAL

13.6 PREPARATION OF TRAFFIC FIGURES FOR USE WITH

OTHER DEPARTMENTAL PUBLICATIONS

REFERENCES CHAPTER 13

Traffic Appraisal Manual

Augus t 1991


2/26

CHAPTER 13 : OPERATIONAL APPRAISAL

13.1 GENERAL

13.1.1 Operational Appraisal is a detailed form of Traffic Appraisal which is needed

particularly in urban areas. It has two main applications. Firstly, during early stages

of building a traffic model it is necessary to examine model output to ensure that

results are reasonable. Later on in a study, the operational appraisal should highlight

areas where a traffic model (and COBA) is oversimplified so that results from the

model can be qualified where appropriate.

13.1.2 Secondly, the operational appraisal should describe the local impact of a

scheme so that the strengths and weaknesses of a proposal can be identified (eg are

any junctions likely to become overloaded ?I.

This will often suggest beneficial

amendments to the detailed design such as ancillary traffic management.

The

appraisal will also identify areas where complementary action will be needed by

statutory and other bodies such as Local Authorities or bus operators.

13.1.3 The first section of this chapter sets out a check-list of operational features

which may be relevant in examining a particular scheme.

The second section

describes methods which will be of value in examining the model itself, as well as in

assisting understanding of traffic behaviour. The third and fourth sections concern

networks and junctions, the latter usually being the kernel of most traffic problems.

The final section describes the preparation of traffic figures for use with other

Departmental publications and discusses some of the problems inherent at these

interfaces.


13-1

August 1991


3/26

13 2 EXAMINING

THE OPERATIONAL FEATURES OF A SCHEME

13.2.1 Most features requiring consideration in operational appraisal lie within the

vicinity of a scheme.

Occasionally, remote features such as signing or an estimate

of increased loadings on unsuitable link roads will require consideration. No check-

list of features requiring examination can be exhaustive because the nature of

operational appraisal is its case dependence.

However the following check-list

contains some common features for consideration many of which can be examined

using the tools described later in 13.3 (a full list of references relating to design is

given in DTp Circular 4/79, ref 1).

i)

Safety:

is this option, in its detailed layout, the most cost effective

which can be designed

? What are the accident costs estimated by COBA?

Could small changes in layout and signing encourage greater volumes of

traffic to use roads with lower accident rates or pass through fewer junctions?

Can pedestrians be further segregated from traffic?

ii) Network balance: for example, does the relief of one bottleneck (which

is not part of a comprehensive route improvement programme) cause longer

queues at the next bottleneck thereby reducing the peak period economic

benefits calculated by COBA? Do climbing lanes following a long stretch of

single carriageway with restricted overtaking provide considerable benefits

which are underestimated by COBA?

iii)

Pedestrians: Will pedestrian facilities be necessary that will alter the

attractiveness of a scheme? For example, a new pelican crossing on a new

dual carriageway (thereby negating much benefit); or a new footbridge or

subway (with the associated capital cost).

iv) Interacting Junctions:

Does the pattern of release of flow from one

junction affect another downstream ? (This can be a positive or a negative

effect; it can be positive particularly when the downstream junction is of the

major/minor type and the minor road flow needs breaks in the major road

flow).

v)

Access: Are there any restrictions (eg height, weight, turning radii,

traffic management policies) which prevent traffic from using the modelled

routes? Should some accesses be closed for safety or capacity reasons? Does

the scheme attract traffic down undesirable routes to gain access to it? (eg

residential areas, unsuitable country lanes, town centre etc).

vi) Junctions:

Which junctions, if any, are likely to become overloaded?

(Junctions which are likely to become overloaded should be identified in the

traffic model).

vii)

Planning Policies: Does the scheme close or assist land development

options?


13-3

August 1991


4/26

viii) Enforcement: Are there any traffic orders (clearways, banned turns,

parking, access restrictions,

traffic signals, speed limits, etc) which are

essential to the viability of the scheme (or a staged opening), for ‘which police

agreement is required?

ix)

Maintenance: What are the maintenance implications? What traffic

costs will be incurred during maintenance

? Maintenance considerations are

a major item in decisions on option viability and standards of carriageways

and junction provision. (As traffic continues to grow, and the age of the road

network increases, maintenance considerations are increasingly important).

x)

Staged Onenings: Does a temporary terminal point, which may last

many years, have adequate capacity to cope? Do any of the previous nine

points have relevance to temporary stages?

13.2.2 When carrying out operational appraisal on new road schemes, it is important

to distinguish between those operational features which fundamentally affect the

decision on the viability of a scheme (eg demolition of buildings to provide necessary

junction capacity); and those features which are marginal details that can be

amended at low cost after a scheme has settled in (eg traffic signal settings).


13-4

August 1991


5/26

13.3

THE TOOLS OF OPERATIONAL APPRAISAL

13.3.1 There is a range of diagnostic quantities which can

be used in appraising the

impact of a scheme.

Nearly all can be extracted from a

traffic model and are of

assistance in understanding, and explaining to others

see

Chapter 151, what a

particular scheme would achieve.

They are also of value in optimising detailed

aspects of particular solutions but, when a choice between alternative solutions is to

be made, they must take second place to the formal economic and environmental

appraisals.

13.32 Operational appraisal is by its nature scheme dependent and the responsibility

of local practitioners.

The following items are amongst those of value:

i)

J ournev Times: The estimated journey time for a vehicle using the

whole of a new route as against the old. For large schemes this can be done

for major sub routes. Figure 13.1 shows how a map can be prepared in critical

sections of a network showing speeds.

ii)

Maior Volumes: Where a scheme is on a route between two major

towns, the volume of traffic between those two towns can be shown. Desire

Line diagrams (figure 13.2) showing the origins and destinations of major flows

can be plotted (program RDSELC): best results are obtained by grouping

traffic into no more than 20 sectors containing zones of homogeneous traffic

interest.

iii) “Select link plots” (figure 13.3) as produced by programs RDSELC and

RDPLOT, showing the traffic contribution to the network of traffic on a

chosen link (usually a major scheme link or a link of interest such as a High

Street.) Separate vehicle classes (eg commercial vehicles) or purpose types

can be examined if appropriate.

iv)

Estimated loadinps that major links may experience over their

economic life can be plotted as in Figure 13.4. This figure can be extended

to illustrate the uncertain opening of a neighbouring link (Figure 13.5). The

base need not be AADT: it could, for example, be a COBA flow group.

v)

Traffic Routes: The routes to certain selected origins (Figure 13.6)

used by the model in assignment can be plotted (programs RDTREE and

RDPLOT). Centres of major towns, terminal points of routes, or potential

sites of major interchanges are commonly chosen.

vi)

Junctions:

A broad estimate of the capacity of any junctions in the

vicinity of the scheme which may become overloaded.

(Junctions are

discussed in 13.5).

vii) Economic Diagnostics: Use of NETBEN AND MATBEN (see Chapter

14) to illustrate which links, and which movements through the network are

gaining respectively benefits and disbenef its.


13-5

August 1991


6/26

viii) Network Comparisons:

Two loaded networks (eg do minimum and do

something) can be compared and plotted such that all links whose volumes

change by more than a specified amount can be identified (programs

RDCOMP, RDFLOW and RDPLOT). This is a very powerful tool for larger

network models.

Traf fit Appraisal Manual

13-6

August 1991


7/26

FI G 13 l

JOURNEY TIME MAP

MEAN JOURNEY SPEEDS

(IN BOTH DIRECTIONS BETWEEN

SECTION LIMITS)

Less than

8 mph

10-12 mph

I

12-16mp h

-

f

16-20m p h

L---_-j

Mc?re than 20 m p h

0 Sectton limit

c h_ Journey tame on sectlon

II-I direct IOn shown m mmutes

1

Not

to Scale


13-7

August 1991


8/26

T

a

c

A

a

s

M

a

1

8

A

1


9/26

FIG. 13.5

SCHEME AND SECTION: M999 BARNBURY- MIDDLEWAYfINCL A777 OPENING OPTION)

lOAD CLASSIFICATION MAIN -URBAN

STANDARD: 02 ALL PURPOSE WITH AT GRADE JIJNCTION

NO HARDSHOULDERS

r

/

.

.

/

HC

32000.

31

30000

29000

28000

27000

2bOOO

35000

23000

?:I000

22000

2 1000

20000

19000

I8000

17000

16000

~~~_-_____-----------

”

HrI I

Urlcertai.

openlncj

LOW

nn


10/26

i

BROOKFIELD

CROFTDOWN

\C RCH wAYo

KENTISH


13-10

August 1991


11/26


12/26

FIG. 13.4

SCHEME AND SECTION: M999 BARNBURY - M

I

ROAD CLASSIFICATION : MAIN - URBAN

STANDARD

32000

31000

30000

29000

28000

27000

26000

25000

24000

23000

2 2000

21000

20000

19000

18000

17000

16000

15000

l

DDLEWAY (NO ASSOCIATED A7771

02 ALL PURPOSE WITH AT GRADE JUNCTIONS.

NO HARDSHOULDERS

I I

1

I I

1

1

‘5

1980

1985

1990

1995

2000

2005

2010

YEAR

-


13/26

13 4

THE USE OF CORDON ISOLATION TO EXAMINE CONGESTED NETWORKS

13.4.1 In all traffic models, except the very smallest of less than about 30 zones, the

use of cordon isolation software to extract a sub-network from the model being used

can provide a powerful tool for the appraisal of difficult areas such as sections of

congested network. A cordon matrix can be extracted in ROADWAY using RDSELC.

A cordon network extraction program may be available in 1981.

13.4.2 The cordon isolation should be as small as is feasible to surround the problem

area. Small models with less than 30 zones or 60 links are highly intelligible and

controllable for detailed operational appraisals and embrace all the major influence

of the usual features under examination (eg one or two bottlenecks; interacting

junctions; a new land use development; terminal arrangements of a new road) which

any model is capable of discerning. This is because traffic quickly, but not instantly,

disperses over the network (see Fig 13.7) and the difference in delays to traffic at

a junction approach at varying traffic loadings is very small providing the junction

approach is working a little below capacity.

If a junction approach is working near

capacity and above, the rate of change of delay with flow is very fast, and so very

small changes in flow at junctions well remote from the influence of the scheme can

show up as major effects in a traffic model. In reality many minor adjustments take

place in traffic demand (ref 2) which remove the major operational problems

predicted by traffic models at small flow changes (these small changes in traffic

demand do of course carry associated costs to traffic). The analysis of congested

networks is described in the TRAFFICQ user manual (see Appendix 13.1).

13.4.3 The time periods for use in such appraisals were discussed in 5.7. Any

factored daily matrix (see also 6.10) which will place the design under feasible peak

loadings of the type of interest in the area of interest will be suitable. (There is an

analogy here with the load testing of structures). Factoring to make the design

respond under directional loadings may be injected if required. For small matrices,

ad hoc methods of factoring the daily matrix will be sufficient: for example, a factor

based on the ratio of peak period flow in the direction being considered to the daily

flow on cordon crossing points.

13.4.4 The following two methods are among the approaches suitable for larger

matrices. In the first, a peak period matrix may be built at, or compressed to, a very

coarse sector level (perhaps 5 sectors of homogeneous traffic interest) and be divided

by the daily matrix compressed to the same level: this then gives a directional peak

to daily factor for each sector. Each cell in the uncompressed daily matrix can then

be factored by the appropriate sector factor (eg there will be 25 factors for a 5

sector system). This coarse sectoring process overcomes peak period sampling

problems because the factors are drawn from a data base which contains sufficient

interviews during the peak period to derive a directional relationship with the daily

matrix.


13-13

August 1991


14/26

DISPERSAL OF TRAFFIC ONTO A LOCAL NETWORK FROM

A ROAD TERMINAL ASSUMING EQUAL TURNING MOVEMENTS

‘A3

Q

I


15/26

13.4.5 A second method which can be adopted, which is more convenient to apply to

synthetic models than to observed data models, is to take “slices” of daily purpose

matrices and assemble these into a peak period matrix. In this method, the data base

is examined to establish the percentage and direction of trips from each purpose in

the time period of interest (and this is sometimes only done for home based journey

to work trips). For example, in the morning peak period a substantial percentage of

the home based journey to work trips will take place, travelling to work. The

morning peak matrix might therefore be of the form:

am peak = al G/A journey to work + a* G/AT journey to work

+

bl G/A home based other + b2 G/AT home based

other

+

. .

where G/A =

G/AT =

. . .

generation/attraction matrix

G/A transposed

and al .> a2

This method also overcomes peak

period sampling problems.

13.4.6 The choice of model to be

used once a cordon matrix and network has been

extracted is quite wide (see also 5.3):

i)

ii)

iii)

iv)

Manual or Semi-Automatic Methods.

The Cordon Matrix and Cordon Network Model.

TRAFFICQ.

CONTFZAM.

Manual or Semi-Automatic Methods

13.4.7 Manual assignment, or the use of the diversion curve in 9.6 together with

calculations of junction delays based on the calculations described in the COBA

Manual Section 6.

The Cordon Matrix and Cordon Network Model

13.4.8 This model will generally require minor

refitting. It may be necessary to

improve the level of model detail within the cordon by means of increased zone and

network definition and the use of more complex assignment procedures (ie capacity

restraint, multi-routeing). If Burrell multi-routing is used, a substantial number of

trees per origin (perhaps up to 100) may be needed with a small number of zones (eg

30) to achieve stability (see 9.5).


13-15

August 1991


16/26

Speed-flow curves can have substantial shortcomings in capacity restraint at this

level of detail: because they are link based, the sum of flows on links entering a

junction estimated by the model may easily exceed finite junction capacities. This

can be partially overcome by the use of notional links describing junction delays at

critical junctions: delays are then calculated at each iteration considering each

junction as a whole.

Several commercially available transportation suites do this

mechanically but for small networks manual intervention to undertake these

calculations can speed up convergence and be profitable in interpreting the behaviour

of the network. Manual intervention becomes too time-consuming for networks with

greater than about five critical junctions.

TWFICQ

13.4.9 TRAFFICQ (ref 3) was outlined in section 5.5. TRAFFICQ operates at the

most detailed level of techniques widely used in the UK, by following individual

vehicles through the network and registering their progress through queues; waiting

for traffic signals to turn green; waiting for gaps to execute right turns; and so on.

For a given traffic demand in a small congested network, TRAFFICQ is suitable for

examination of:

i)

a new road;

ii)

road widenings or flaring of junction approaches or exits;

iii)

change in form of junction control or layout;

iv)

change in location of a pedestrian facility;

v)

introduction of bus lanes, banned turns, one-way systems etc;

vi)

the effect of changed traffic demand or behaviour - say from a new

industrial estate, superstore, or car park.

13.4.10

One of the virtues of using TRAFFICQ for small networks is that whilst

all route choice options (including multi-routeing) are open, a full route choice model

does not have to be fitted because manual assignment is used (diversion curves, see

9.6, or other methods external to the program can of course be used): this makes for

very fast appraisal. However this property ceases to be a virtue on networks much

larger than 30-50 links. The detail output by TRAFFICQ in networks over the 50 link

size is also too large to be examined critically.

13.4.11

TRAFFICQ is supported on behalf of the Department through the

arrangements described in Appendix 13.1. A comprehensive user and applications

manual is available.


13-16

August 1991


17/26

CONTRAM

134.12

CONTRAM which was also outlined in 5.5, is a dynamic traffic model

developed by TRRL (ref 4) which embraces route choice as an important feature of

the program.

The only extension in data requirements over its conventional steady

state capacity restraint counterparts is the need to provide:

i) details of junctions; and

ii) origin and destination movements with associated times of departure

from the origin.

For operational appraisal this associated time of departure can either be measured,

or constructed using a flow profile, or simply assumed to be constant over the time

period being considered.

13.4.13

An advantage of CONTRAM is that the program has undergone

calibration and validation tests using purposively collected data rather than relying

solely on “Case Law” generally used in traffic models.

13.4.14 In one study CONTRAM has been used to investigate traffic delays

during maintenance operations.


13-17

August 1991


18/26

13.5 JUNCTION APPRAISAL

General

13.5.1 Junctions, of whatever type, are the kernel of most operational problems and

almost exclusively so in urban areas.

There are 4 major types of junction:

i)

grade separation;

ii)

traf fit signals;

iii)

roundabouts; and

iv)

major/minor junctions

Similar problems arise at the discontinuities:

i)

climbing lanes and lane drops;

ii) pedestrian crossings (zebra and pelican);

iii)

level crossings;

and at the system solutions such as gyrator&s, G-turns and Q-turns which generally

aim at re-routeing right turning traffic.

13.5.2 Well-chosen and well-designed junctions can provide very great benefits at low

cost. For example, if just one extra lane can be provided at an over-saturated traffic

signal approach, discharging for only 30 seconds per minute, this can remove queuing

vehicles from the approach at a typical rate of 5 kilometres of queue per hour.

13.5.3 The major operational features of the junction types can be summarised as

follows:

i)

Grade Senaration: varies in scale from a simple half diamond up to a

fully free flowing interchange (eg Almondsbury M4/M5); effective in reducing

accidents and delays; can be intrusive with large land take and cause

severance; construction can cause considerable disruption, and the diversion

of statutory undertakers’ equipment in urban areas can be an insurmountable

problem; provision for pedestrians may involve them in considerable detours.

ii)

Traffic Signals: mainly applicable to existing urban junctions where

other possible solutions would involve property demolition; cope well with

heavy flows with small turning movements; more delay incurred outside peak

hours than with other junction types; not as safe as roundabouts, particularly

on high speed roads; maintenance can be a problem but the facility for

varying timings gives operational flexibility;

can be useful in area control

policies as junctions can be linked; adaption for pedestrians fairly easy.


13-19

August 1991


19/26

iii)

Roundabouts: vary in size but provide a good solution for moderate to

high flows in both urban and rural situations; especially good in suburban

areas and where heavy turning movements occur; safest form of at grade

junction;

in rural areas deflection of through vehicle paths normally

determine the size as opposed to flow considerations; in urban areas heavy

goods vehicle characteristics can determine minimum size; all through-traffic

delayed;

can cause problems when associated with linked urban traffic

signals; may need complementary pedestrian facilities.

iv) Maior/minor nrioritv junctions:

the most common form of junction;

suitable for low to moderate flows;

not as safe as traffic signals or

roundabouts;

there are three standard types - simple T, ghost islands

and

single lane dualling; on single carriageways, ghost islands and lane separation

can reduce overtaking opportunities; ghost islands in the mouth of the minor

road and physical and ghost islands on the major road decrease accidents;

main road through traffic is not delayed.

v)

Hybrids and others: used only to overcome specific site problems; can

be difficult to sign and can cause access problems.

Cauacitv and Delay

13.5.4 Any formula for assessing delay is only as good as the information on capacity

and traffic flows fed into it. Very small changes in either when a junction approach

is operating near or above capacity will have substantial impact on results. A

junction approach has three identifiable states of operation:

i)

under canacitv

(the steady state condition where the arrival of traffic during one interval has

no impact on traffic arriving in subsequent intervals);

ii) around cauacitv

(the arrival rates of traffic during some, but not all, intervals in a larger time

period is greater than the capacity of the junction); and

iii) over canacitv

(the arrival rate of traffic is greater than the capacity of the junction

approach and the queue is growing steadily).

13.5.5 The following data is required to operate the “time dependent” formulae that

can evaluate all three of the states in 13.5.4 above:

i)

the geometry of the junction;

ii) the arrival flow during an interval at each junction approach;

iii)

the capacity at each junction approach for the interval;

iv)

the queue at each junction approach at the start of the interval; and


13-20

August 1991


20/26

VI

the nature of arrivals and departures from the junction (eg random

arrivals, regular departures).

13.5.6 A version of these time dependent formulae is now a component part of the

following programs (see also section 6 of COBAS Manual):

i)

COBA

ii) ARCADY

iii)

PICADY

iv)

MIDAS

The formulae are applied in two forms, “high definition” or “low definition”,

depending on the type of appraisal.

Strategic appraisals such as COBA cannot

realistically estimate detailed 5-minute by 5-minute flows for new roads over 30

years: in these cases the formulae are applied in low definition using a “block time”

based on average surveyed profiles to allow representation of the build up and decay

of traffic.

13.5.7 ARCADY (refs 5 and 6) and PICADY (ref 6) are programs requiring input in

short time intervals which output queue lengths and average delays over these

intervals: ARCADY is concerned with isolated roundabouts and PICADY with isolated

major/minor junctions.

Both these programs are powerful tools for understanding

how junctions operate and therefore for both improving existing junctions and

allowing improved advice to be issued on new designs. Because of the uncertainty

of forecast future year turning movements at new junctions, they cannot be directly

applied in these cases without careful formulation of the design problem (see

13.5.10).

13.5.8 MIDAS (ref 6) allows an independent “low definition” economic appraisal of

junction types and is more flexible than the junction economic appraisal included in

COBAS. It does not, however, consider all the factors necessary to allow a full

assessment of junction choice.

13.5.9 Section 6 of the COBA manual describes calculation of delays at junctions in

more detail.

Estimation of flows at junctions from a Traffic Model

13.5.10

Traffic models cannot, in general, directly provide reliable estimates

of the forecast year peak period turning movements which may be required in design

publications. The traffic model, in’an area with fully modelled movements, will

usually be adequate to estimate the following (in descending order of accuracy) for

the high and low growth estimates:

i)

the sum of the approach flows to the junction in AADT;


13-21

August 1991


21/26

ii)

the two way link flow on each of the significant junction arms in

AADT; and

iii)

the balance of the turning movements at the junction. For example,

for a three arm major/minor junction, considering the minor arm, either

a.

all movements in and out equal; or

b.

left in, right out dominant; or

C

left out, right in dominant.

13.5.11

It is therefore recommended that the turning movements used for

junction design are not those output directly from the traffic model but those of the

approach flows apportioned to reflect dominance and symmetry (unless there are

good reasons for non-symmetrical flows) factored to the peak values required for

design (see section 13.6). For example, in iii)b above, having established the

dominance, for design purposes the minor arm flow might be apportioned 66 to the

right turning movement.

Similarly, the apportionment for iii)c above might be 33

of the minor arm flow to the right turning movement.


13-Z

August 1991


22/26

13 6 PREPARATION OF TRAFFIC FIGURES FOR USE WITH OTHER

DEZPARTMENTAL PUBLICATIONS

General

13.6.1 The existing range of Departmental publications for use in design which

require estimates of traffic volumes are given in ref 1. These publications cover:

- economic appraisal

- calculation of road traffic noise

- road pavement design

- geometric design.

In general, these publications have evolved using flow definitions that could be

measured at the present day (eg “highest flow for any specific hour of the week

averaged over any consecutive 13 weeks during the busiest period in the year”). The

definitions of flow, and those of vehicle class, also vary between the publications.

The forecast flows of traffic, however, which traffic engineers can estimate, are

based on parameters which are in the main daily or annually based (eg national road

traffic forecasts).

The publications, many of which are due for replacement for

other reasons, are to be systematically revised as convenient to be consistent with

traffic forecasting methods. This section discusses the flow estimates required from

the traffic appraisal by these publications.

13.6.2 There are two general points to be made. Firstly, the unit of flow with which

all traffic appraisals can sensibly end is 24 hour Annual Average Daily Traffic and

this unit will be adopted by the revised design publications which use forecast flows.

Peak hourly flows can only usually be obtained by factoring (see 5.7). (Appendix D14

contains factors to convert link flows after assignment from the base of a trip matrix

to 24 hour AADT; see also 6.10 and 8.1). The calculation of 24 hour AADT will

depend on which of 3 road type classifications is given to a link as discussed in 6.10.

13.6.3 Secondly, a rigid approach to major investment decisions, such as the choice

of road type, can lead to sub-optimal designs because it takes no account of factors

which may be specific to a particular scheme, ie construction costs, environmental

impact, traffic benefits, accident reductions, delay costs to traffic during

maintenance, plans for the future, and so on.

Departmental publications such as

Departmental Standard TD 9/81 on Highway Link Design have made clear both the

importance of a flexible approach and the role of economic appraisal, but in some

instances the full dimensions of a choice have been insufficiently examined and too

much reliance has been placed on tables and figures relating to operational

characteristics. Clearly, overall value for money, taking into account all the above

factors, must be the determining factor.


13-23

August 1991


23/26

Economic ADDraisal

13.6.4 The COBAS and QUADRO manuals are sister publications to this manual and,

as far as is possible, recommendations, definitions, procedures, factors and data are

consistent between the three manuals.

13.6.5 Attention has been paid to making the following compatible:

i)

road classification;

ii)

count conversion factors;

iii) forecasting parameters;

iv)

vehicle class definition;

VI

speed-flow geometry relationships; and

vi)

flow groups.

It is recommended that those commencing new studies adopt the same standards as

COBAS and TAM wherever possible.

13.6.6 Interface programs from ROADWAY have been produced to pass input files

directly into COBA (see 14.2). COBAS accepts directly the following flows:

12

hour weekday flows (0700-1900)

period models are recommended

models);

in any month (12 hour interviewing

in TAM for new observed-data

16 hour weekday flows (0600-2200 hours) in any month; and

AAHT (Annual Average Hourly Traffic).

Environmental

ADDraiSd

13.6.7 Traffic figures are needed as a basis for the assessment of the main

components of an environmental appraisal.

In all cases high growth figures should

be used. The simplest areas are those of Community Severance and Visual Intrusion.

Here the highest AADF during the 15 years after opening should be used. For Air

Pollution and Driver Stress the applicable figure is the annual average peak hour flow

in the same period coupled with the speeds and HGVs relevant to those flows.

Similar information is needed for noise calculations except that the flows should be

18 hour AAWF in the worst month of the worst year.


13-24

August 1991


24/26

Road Pavement Desipn

13.6.8 Technical Memorandum H6/78 is the current Departmental publication which

updates Road Note 29 (third edition) on pavement design. The traffic estimate

required in the memorandum is the cumulative number of million standard axles (msa)

that will pass over a pavement during its design life. Appendices A, B and C of

H6/78 contain a worked example of msa calculation. The 24 hour average daily

traffic should be taken to be AADT and commercial vehicles to be those vehicles

greater than 30 cwt (1.5 tonnes) unladen weight.

Geometric Design

13.6.9 Most of the geometric design publications use definitions of peak hour flow.

However, peak hour flow forecasts using the centrally available data and forecast

parameters can only be achieved for future years by factoring from daily estimates.

Factors have been prepared using the Department’s Traffic Flow Monitoring sites,

grouped to the road classification system shared by TAM and COBAS, to allow 12

hour flows, AAWT and AADT to be factored to the 30th, 50th, 100th and 200th

highest hour of flow in a year. This has also been done for the peak hourly demand

as defined in Technical Memoranda H6/74, H9/76, Hl8/75 and Hl2/76. These factors

(contained in Appendix D14 with their associated coefficients of variation) can be

used to interpret the operational information contained in memoranda which refer

to peak hourly flows.

13.6.10 Recent work (ref 7) has, however, assisted understanding of the

accuracy and value of peak hour flow estimates obtained from daily traffic

measurements at the present day. In particular:

i)

the peak hour/daily flow ratio (PDR) has been found not sufficiently to

vary between sites to be of the value previously thought;

ii)

factors deriving the 30th highest hourly flow (not dissimilar from peak

hourly demand) from a perfectly known annual flow embrace a 95 confidence

interval from the 10th to the 150th highest hour; and

iii) the definition of peak hourly demand, which has been found in practice

difficult to estimate, does not have superior qualities of stability over a 30th

highest hour.

Emphasis is now placed by the Department on 24 hour AADT (for example, as in the

Design Standards of Highway Link Design - TD9/81) so that the basic traffic unit for

the economic appraisal, traffic appraisal,

and operational appraisal become

consistent.


13-25 August 1991


25/26

Traffic Signals

13.6.11

The criteria for traffic signals at junctions are described in Circular

Roads 5/73 and technical memorandum H1/73. The flow unit used is the four busiest

hours in a day.

Traffic signals can however be reset after installation based on

observation and estimates of future volumes for setting signals is therefore not

critical. The criteria for signal installation are not solely a matter of estimating

future traffic volumes but where these are important, the factors converting 24 hour

AADT to the second highest flow group in COBA may be of value.


13-26

August 1991


26/26

REFERENCES CHAPTER13

1.

2.

3.

4.

5.

6.

7.

DTp Highways and Bridges Departmental Standards and Advice Notes -

Technical memoranda - 1983 Numerical Index, May 1981.

Dawson J A L: “Comprehensive Traffic Management in York: the Monitoring

and Modelling”, Traffic Engineering and Control, October 1979.

Logie D M W: “TRAFFICQ: A Comprehensive Model for Traffic Management

Schemes”, Traf fit Engineering and Control, November 1979.

Leonard D R, Tough J B and Baguley P C: “CONTRAM: A Traffic

Assignment Model for Predicting Flows and Queues During Peak Periods”,

TRRL LR841, 1978.

Advice Note on The Effect of Entry-Circulation Flow Based Capacity Methods

of Roundabout Selection and Design; also New Visibility Criteria, Department

of Transport, RLT Directorate, May 1981.

User Manual for Junction Appraisal Programs ARCADY, PICADY and MIDAS,

Department of Transport, Highway Engineering Computer Branch, 1981.

Machin H A:

“Design Parameters for Rural Roads and their Estimation”

(unpublished), STG Division, DTp, 1977.

E-4-2 Traffic Appraisal Manual, Chapter 13

Documents

Transcript of E-4-2 Traffic Appraisal Manual, Chapter 13