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[email protected] Paper from: Urban Transport VIII, LJ Sucharov and CA Brebbia (Editors). ISBN 1-85312-905-4
Evaluating access road network structures for built-up areas from a sustainable transport perspective J. Krabbenbos, M. F.A.M. van Maarseveen & M.H.P. Zuidgeest Department of Transportation Engineering & Management, Universi& of Twente, the Netherlands.
Abstract The relation between motor traftic and livability in residential built-up areas is usually rather strained. Separation of terminating traffic from through-traffic by infrastmctural measures is often difl%xdt to realise without causing a decline in the accessibility of the area. On the other han~ a good accessibility might generate substantial through-t.raflIc with all its adverse impacts on livability and traffic safety. Within the framework of the sustainable safety program, a study has been carried out to analyse the impact of various elements of a road network structure in a built-up area on accessibility, livability and traflic safety. The elements of the network structure are: the number and distribution of access roads. the number and distribution of access directions, and the type of linking to the external road network. A simulation study for a large number of synthetic network structures shows that the impact of access directions is much larger than that of access roads, since it aflkcts route choice more substantially. In additio~ the amount of through-trat%c is very sensitive to the type of linking to the external network. The simulation study clearly demommates the divergent and partly opposite impacts on accessibility, livability and traflic safety. The relationship between the number of access directions and accessibility has also beerI tested in an empirical case study for four different built-up areas in the city of Enschede, the Netherlands. The results of the field study confirm the simulation results. The implications for the (re)design of access road network structures for built-up areas are discussed. In addition, recommendations for t%ture research are provided.
© 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email
[email protected] Paper from: Urban Transport VIII, LJ Sucharov and CA Brebbia (Editors). ISBN 1-85312-905-4
1 Introduction Notions ofsustainable development have evolved in recent years preceded by the appearance of the ‘Our Common Future’ report (WCED [1]) and international agreement conform ‘Agenda 21‘ (UNCED, [2]). A wide array of issues and public policy concerns resulted. In the transportation field the concept of sustainable development gave rise to conceptions as sustainable transportation, sustainable cities etc. Topics as the role of transportation in ensuring future availability of petrol, urban air pollution, traffic congestion and traffic safety are addressed frequently since. Liveability of residential built-up areas is closely comected to discussions on sustainability. It is here, where the dichotomous concept of sustainable development, indicating an apparent, inherent contradiction, becomes clear. On the one hand accessibility is highly valued, indicating the ease of physical movement in built-up areas, whereas on the other hand there is the search for a liveable environment in built-up areas, in other words a built-up area with limited danger or nuisance by motorised traffic, a place where children can safely play and cross streets and a place where social interaction is possible etc. The residential street environment is a matter of continual concern to residents living in it, but of only fleeting interest to the passing driver. Therefore, there is a strong argument that it should be designed in a way that living and driving are in balance. It is only then that jiinctzon - ~orrn - use of the built-up area are in balance for both drivers as well as people residing in the area. This paper discusses the relationship between characteristics of the residential transportation network versus its performance in terms of livability, safety and accessibility. First traffic liveability is discussed based on the conceptual ideas preceding this research, followed by a simulation study, where elements of road structures in a built-up area are systematically varied. In addition, the ideas have been tested in a case study for the city of Enschede, the Netherlands. Finally, some lessons are drawn in the conclusions and recommendations.
2 Characteristics
of built-up areas
2.1 Traffic livability Liveability is a very wide concept. It refers to human well-being, In this research, the term livability is narrowed to traffic liveability, which is indicated in terms of livability, accessibility and traffic safety (figure 1). Liveability then refers to the amount of through-traffic and the crossability of streets. From the perspective of live ability, through-traffic in built-up areas is undesirable. Air pollution, noise nuisance and perceived traffic unsafety caused by traffic have a negative influence on the well-being of the local residents in the built-up area. Even though all traffic influences livability, through-traffic can be seen as avoidable traffic in built-up areas in particular. In addition, liveability refers to the crossability of streets. In order to be able to cross a street, a minimum acceptable gap in the traffic flow of at least the
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Tronspo~t in [he 21st Centzvy
271
crossing time needs to occur. As traffic flow increases such gaps occur less often. The crossability decreases at two-direction flows of more than 500 velu% (ASVV [3]). Furthermore, Miedema and Van der Molen [4] demonstrated that lower traffic flows influence the experience of the residential environment positively. Accessibility of a built-up area can be derived from the average car trip length, relative to the size of the built-up area. The average car tip length denotes the generalised cost of reaching the point of destination from the road surrounding the built-up area. Traffic safety is the product of exposure and risk. Exposure is derived from traffic performance, which can be expressed in vehicle kilometres. The roads surrounding the built-up area have different characteristics than streets in builtup areas, therefore risk on roads and streets differ.
through-traffic
livability
— crossability
accessibility
I
traffic
traffic performance
risk I
Figure 1: Elements of traffic livability 2.2 Elements of road network structures In this study, elements of a road network structure that influence the route choice of motor traffic have been taken into account, These elements are: (1) the number and distribution of access roads, (2) the number and distribution of access directions, and (3) the type of linking to the external road network (Janssen [5], Cerwenka [6], Van Minnen [7]). These elements are illustrated in figure 2. For structures (1 – 4) the type of linking is kept constant at the comers, i.e. traffic enters the network structure in the comers instead of centrally, as in structure (5), For example, figures 2-2A and 2-2B have an equal number of access roads, i.e. 2, but differ in the direction, i.e. West and South vs. North and South, The impact of these elements has been analysed systematically in a theoretical study.
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da-
@
I-t-t-+] (5)
@
(2A)
I
(3)
I
Fieure 2: The number and distribution of access roads and directions for comer. type of linking (l-4) to the external road network as well as centrally (5).
3 Simulation study 3.1 Simulation lay-out To assess the impact of ( 1) the number and distribution of access roads, (2) the number and distribution of access directions and (3) the me of linking to the external road network, a simulation study was earned out (Krabbenbos [8]). The microscopic simulation model Getrarn/AIMSUN2 [9] has been used. In the model, a hypothetical built-up area, including the outer (surrounding) road structure was built. A square built-up area with a grid road structure of about 41 hectares was simulated. The housing density and the trip generation were assumed to be equally divided over the built-up area. The basic premise in the route assignment is the assumption of a rational traveller, i.e. one choosing the route which offers the least perceived travel time, as in the so-called Ist equilibrium of Wardrop (see Orhizar and Willumsen [10]), so delays caused by congested traffic may affect route choice. Therefore, two scenarios, one with a low level of congestion at the outer structure and one with a high level of congestion at the outer structure were carried out. A large number of synthetic network structures were simulated while systematically varying the elements of road structures.
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X73
3.2 Simulation results Figure 3 shows one of the results for the high level of congestion scenario. Because of the congested situation at the outer structure one can expect throughtraffic within the built-up area. In this example the linking to the external road network is in the comers of the network structure as in figure 2 (structures I-4).
v
~oo__–.-——— ./.— /—-400
/.–
_ ,.=-.
_,-
-~
300 zoo
____ .~
—— __ 1- ——— -———— —. ————-.. __ —y ———— ———
7
./
-. 1vu
------~
o
--
No. of access dkections
w-~
2aT2
No. of access roads
Figure 3: Amount of through-traffic related to the number of access directions and number of access roads. The simulation study shows that: ● The impact of access directions is much larger than the effect of the number of access roads, since it affects route choice more substantially. The addition of an extra access road, preferably in a new direction and an equal distribution of the access roads over the directions appears to be most effective in terms of access; ● The amount of through-traffic is very sensitive to the type of linking to the external network. Congested traffic causes delays making built-up areas an attractive alternative for through-traffic. Too much through-traffic has a negative influence on liveability; ● The crossability of streets is another measure for livability. Avoiding through-traffic increases crossability of streets. Adding more access roads, preferably in different directions and an even distribution over the sides of the built-up area, divides traffic more equally over streets. A side-effect of building more access roads can increase through-traffic, which influences crossability in a negative way; ● Accessibility refers only to trips terminating in the built-up area and is therefore independent of the outer network. Accessibility increases by
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[email protected] Paper from: Urban Transport VIII, LJ Sucharov and CA Brebbia (Editors). ISBN 1-85312-905-4
increasing the number of access directions, the number of access roads and an equal distribution of these access roads.
To estimate the effect of road network structure on traffic safety, not only the built-up area has been taken into account, but also the surrounding structure. Increasing the number of access roads may decrease trip length and therefore traffic performance. But if this leads to more through-traffic in the built-up area, risk can increase, as function – form – use of the streets are not balanced anymore. As the relation between flow and risk is depending on the detailed situation and external factors a general conclusion concerning the influence on traffic safety cannot be given. Although traffic safety has been researched in this study, the results are not discussed here, as ilu-ther research needs to be done to estimate the relation between flow and risk on streets and roads more exactly. For the preliminary results the reader is referred to Krabbenbos [8]. The simulation study clearly demonstrates the divergent and partly opposite impacts on accessibility, livability and traffic safety. More access roads, preferably distributed equally over the various sides of the built-up area, increase the crossability of streets and the accessibility of destinations in the area, The amount of through-traffic and traffic safety restrains the maximum number of access roads. As every situation is unique, it is not possible to give general conclusions concerning the optimal number and distribution of access roads and access directions, because this also depends on the other factors like the internal road structure and the amount of through-traffic around the built-up area (the external road structure). The amount of through-traffic is very sensitive to the type of linking to the external network.
4 Case study 4.1 Introduction The relationship between the number of access directions and accessibility has been tested in an empirical case study. Therefore a comparative study has been carried out. Four built-up areas in the city of Enschede, the Netherlands have been selected. These built-up areas have respectively one, two, three and four access directions. Figure 4 shows the road network structures of the selected areas.
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1‘rb~inTk7nsport in the 2 ht Centun
Z1.5
Ei (a)
(b)
(c)
(d)
Figure 4: Road network structures of the selected built-up areas in Enschede: (a) Voskamp, (b) Stroinkslanden, (c) Wesselerbrink, (d) ‘t Zeggelt, with numbers of access directions varying from 1 to 4. 4.2 Method In order to assess therelationship beWeenthe number ofaccess directions and accessibility, accessibility is defined as the distance inhabitants of the built-up area have to travel in order to reach their point of destination, A method has been developed to derive this distance. It is measured by the internal car trip length (L~iP), which is the distance travelled on streets by terminating traffic. The average internal trip length was derived using the following formula:
(1)
with: Ltrip
= internal
L1
= length of street link i
car trip length
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276 1‘il~(ltl ii
N
Tratq]ort
in the 21st (“CH(ZO:I
= flow on street link i = number of terminating
trips
4.3 Data collection The length of the streets (LJ was derived from the National Road Database. Flows on streets and the number of through-traffic vehicles were obtained by canying out a traffic counting survey. To make a distinction between terminating traffic and through-traffic, a Iicence plate survey was performed, The data was collected on workdays (Monday, Tuesday and Thursday) between 15:30 and 17:30. 4.4 Results For every built-up area, the average trip length for terminating traffic was calculated. Due to differences in size (A) of the areas the average trip length (Lti,) has been adjusted by dividing the trip length by the root of the size of the built-up area (Lm#A) in order to make the results comparable. This results in an adjusted average internal trip length (table 1). In figure 5 the adjusted internal car trip length for terminating traffic is shown graphically. Table 1. Characteristics
access directions access roads area size (A) LtiiP unadjusted [m] LtilP adjusted [-]
of built-up areas and the average internal car trip length. Voskamp 1 1 9 447 146
,%roinkslanden 2 2 28 574
Wesselerbrink 3 3
114
‘t Zeggelt 4 9
40
28
838
414
88
80
16(I 140
120 100 80 60 40 20 0 Voskamp
Stroinkslanden
Wesselerbrink
‘t Zeggelt
Figure 5: Adjusted average internal car trip length.
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[’than lmnsport
itl the 21s[ Century
277
The results show that, after adjusting for the size of the built-up area, the average internal car trip length for terminating traffic decreases with the number of access directions, The results of the field study confm the simulation results.
5 Conclusions
and recommendations
As the number of possible network structures is unlimited in practice, it is not possible to give the optimal number and distribution of access roads and access directions for the different structures. However, this research hopes to have revealed at least the notion that liveability and accessibility of built-up areas are sensitive to the type of linking to the external network, the amount and direction of access roads and the congestion level at the outer structure. It has been shown that the effect of the direction of the access roads is more important than the number of access roads. Furthermore, the amount of throughtraffic is highly dependent on the amount of access roads, since through-routes are shortened. On the contrary, an increase in the number of access roads and an equal distribution in the direction improve the crossability of the streets, because of a more homogeneous distribution of traffic in the network, and therefore an increased number of minimum acceptable gaps. Most of these simulation results are also revealed in the Enschede case-study. An increase in the number of access roads leads, indeed, to a decrease in the average travel distance for terminating traffic in the built-up area. In order to avoid through-traffic in the built-up areas one could consider increasing speeds at the roads, whereas decreasing speeds at the streets. This could also be done by enlarging the length of built-up area streets and, of course, by lowering the number of access roads. With the results in mind traffic professionals should now be able to see what kind of measures taken to the built-up area network structure as well as to the outer structure have what kind of effect. Examples of these measures for (re)design of access road network structures are given in table 2. Further research should be conducted to the relation between traffic unsafety versus flow on both the streets as well as roads in the area under consideration.
6 Acknowledgements The authors wish to express their gratitude to the Institute for Road Safety Research (SWOV), Leidschendam, the Netherlands for their valuable remarks and help in the research, as well as the fiiancial means to conduct the casestudy. Furthermore, the traffic department of the Municipality of Enschede is acknowledged for their help in the case-study.
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Table 2. Examples of measures taken to decrease the number of through-traffic. Measure Increase speed at roads
Deerease speed at streets
Increase length of through -route
Method Improve throughput at major intersections Divert through-~~lc Speed measures
Examples . Optimise Mlc control design ● Roundabout ● Ring road ● TratHc humps ● Narro~ streets ● Intelligent Speed Adaptation (ISA)
Change internal network structure Change distribution of access roads compared to the outer stxucture
Decrease number of access directions
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Agenda 21: programme of action for sustainable
development, Rio de
Janeiro, 1992. [3] Information and technology
centre for traftic and transportation (CROW) Recommendations for trafic provisions in built-up areas, Ede, 1998. [4] Miedema, B., Molen, H.H. van der, Veiligheidsbeleving voor en naherinrichting van tien verblij~sgebieden [In Dutch]. VK-8349. Centre
for environmental and tic psychology. University of Groningen, 1983. [5] Janssert, S.T.M.C., Denmnstratieproject herindeling en herinrichting van
stedelijke gebieden (in de gemeenten Rijswijk en Eindhoven) ~n Dutch]. R84-28 I., Institute for road safety research (SWOV), Leidschendaq 1984. [6] Cerwenka, P. & Henning-Hager, U. Verkehrssicherheit in Wohngebieten;
Einfl@r@en Bewertung und P/anungshinweisen [In German]. Forschungsbericht 99, Federal highway research institute (BASt), Bergisch Gladbach, 1994. [7] Minnen, J, van, Geschikte grootte van verblijfigebieden [In Dutch]. R-9925. Institute for road safety research (SWOV), Leidschendam, 1999. [8] Krabbenbos, J. Verkeersleejbaarheid in woongebieden – Een modelstudie naar de relaties tussen kenmerken van ontsluitingsstructuren en verkeersleejbaarheid met toetsing aan de prakfij’k [In Dutch]. M. Sc.-thesis, University of Twente, Enschede, 2000. [9] Getram/AIMSUN2, Transport simulation systems (TSS), Barcelom. [10] Ortimr, J. de D. and Wil&sen, LG., A40delling transport. John Wiley & Sons, Chichester,
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