Impact of permitted driving speed on the design of urban street network in Latvia/ Leidziamojo vaziavimo greicio itaka miesto gatviu tinklo projektavimui Latvijoje/ Atlauta brauksanas atruma ietekme uz pilsetu ielu tikla projektesanu/ Lubatud soidukiiruse moju Lati linnateede vorgu projekteerimisele.

Lazda, Ziedonis ; Smirnovs, Juris

1. Introduction

Different solutions for road infrastructure may greatly influence speed management on roads thus providing positive impact on traffic safety. Results of different studies show that when specific changes are implemented in road infrastructure also the behaviour of road users both in short--and long-term is about to change, as well. For instance, studies of UK Dept of the Environment, Transport and the Regions have shown that in locations where permitted driving speed was changed But no other alterations to infrastructure were implemented the change in actual driving speed was observed for 1/4 of the max permitted driving speed determined with road signs.

Alterations of road infrastructure that are aimed at more harmonic infrastructure and reflect the needs of all road users are important not only as one of techniques for speed management applied by transport administrations, but also because they contribute to the construction of streets and squares in our cities and tOwns that are safer for pedestrians and vulnerable road users and simultaneously ensure more mobility for motorists (Antov et al. 2009; Zilioniene et al. 2010).

2. Road functions and road categories

Substantiation of geometrical parametres for road and street network in urban areas is done on the basis of knowledge about interconnections of transport flows in urban traffic (Cameron, Elvik 2010; Ratkeviciute et al. 2007). The choice of values of a number of dimensions applied in the design of urban road and street network is based on the Latvian construction norms specified in 1992 year LBN 100: Teritorialplanosana. Pilsetu un pagastu izbuve [Territorial Planning. Urban and Town Construction] which in turn is based on the former Russian construction regulations and norms CHUII 1989: [TEXT NOT REPRODUCIBLE IN ASCII] [Building Regulations. Town Planning. Planning and Construction of Urban and Rural Settlements]). The parametres specified in these norms were acquired either in the result of research done in 60-ies to 80-ies on highways outside urban areas or on city highways in sparsely builtup territories or simply taken over from the experience of other countries without any testing in the existing traffic conditions.

2.1. Existing street categories and their basic importance

Categories of city streets in the past were determined according to the classification shown in Table 1 with respect to foreseen transport and pedestrian flows.

It is known that the alteration of road profile with different engineering measures which is aimed at influencing the choice of driving speed is usually done in urban territories, but is rather rare in sparsely built-up areas. Unfortunately the author has observed that in Latvia different infrastructural measures the aim of which is speed management are used rarely even in cities with densely built-up territories. In the middle ages the gates of fortified towns required different behaviour of road users. The idea of gates is also applied in the modern architecture in many cities and towns as one of the most important speed management techniques that creates the need for different road user behaviour in transit zones between the roads outside built-up areas and the roads inside built-up areas where much lower speed is required in order to protect vulnerable road users (Lama et al. 2006; Lazda, Smirnovs 2009).

Traffic calming which is used to physically ensure lower traffic speed and reduce the amount of traffic has a long history, as well. Initial trials were carried out in Radburn (USA) in the end of 20-ies when urban streets and squares were transformed with the aim to reduce the amount of traffic. This approach was further developed in Europe in 60-ies when the first guidelines in Sweden stipulated the development of urban areas with ring-type road systems. This idea spread in other countries, as well. Since then the speed management in urban territories with infrastructural measures became very popular and saved many lives all over the world.

Since 90-ies traffic calming and other speed management techniques are implemented not only in urban areas but also on roads outside urban areas, for example, in Denmark, the Netherlands, Germany and the United Kingdom. Alterations in infrastructure connected with speed management are now widely used for traffic calming on urban streets with many variations suited both for motorways and urban arterial streets that require speed management due to safety problems with increasing high-speed traffic flow.

In 60-ies and 70-ies guidelines for urban planning with specific respect to traffic safety were developed in Sweden. These guidelines recommended the creation of such traffic environment that had simple planning and was easy to comprehend by the road users.

Further concepts were developed in 80-ies and 90-ies that specifically stressed the adequacy of road structure so that the road users could themselves choose appropriate driving speed. As indicates in Organisation for Economic Co-operation and Development, European Conference of Ministers of Transport and OECD/ECMT Transport Research Centre (The JOINT OECD/ECMT) report of 2006 "Speed management" the proposed concepts emphasised the need for structuring the road network and adapting uniform and consequent design principles that would contribute to the reduction of the variability of different road profiles in the road network.

Having analyzed experience of other countries in which climatic weather is similar in Latvia, it is concluded that it would be to use a narrower carriageway lanes than in the existing local design rules. For example, according Roads and Transportation Association of Canada "Geometric Design Guide for Canadian Roads" of 1986 year, lane width for free flow of arterial street having more lanes in each direction is related to design speed 80 km/h, lane width is 3.5 m.

SWOV Institute for Road Safety Research in 1994 report "Infrastructure Design and Road Safety" says that at prevailing speed 70 km/h, which is equivalent to an appropriate design speed of 80 km/h, the width of the lane arterial street is 3.5 m. Besides, the arterial street with distribution function, the width of lane is from 3.0 m to 3.25 m, that is considerably less than the Latvian design standard set (Table 2).

2.2. General road functions

Based on different references the author has come to conclusion that in general the following three basic road functions may be defined:

Traffic flow function.

Roads with traffic flow function may ensure long-distance traffic efficiently. Inter-urban motorways and highways and sometimes urban arterial streets have the flow function. Other modes of transport and vulnerable road users on such roads have to be strictly separated. The number of access and exit points is limited and the distance between crossings is considerable.

Traffic distribution function.

Roads with traffic distribution function provide opportunity for the road users to access and exit from any urban and rural territory in the whole length of the road. Crossings are more frequent and all types of manoeuvres in crossings are allowed. Such roads are also used by different types of public transport.

Traffic destination or access function.

Roads with access function allow actual access to properties in the whole length of the road or street. Both crossings and interchanges ensure traffic exchange. Engineering measures may be needed in order to ensure appropriate low speed.

2.3. General road categories

In order to choose the most appropriate road profile that will greatly influence traffic flow and safety, the road function has to be defined and also the road category has to be determined that would show the location of road (roads in urban areas or outside urban areas) and road type (e.g., motorway) (Table 3).

The general idea is that the roads depending on their function are self explaining. Therefore based on the above mentioned the characteristic cross-profiles and crossing layouts, as well as, type of use and max permitted driving speed have to be determined according to road function types. In addition to that, the placement of horizontal and vertical road furniture has to conform to the main road function and followingly the determined appropriate speed.

2.4. Design speed

Depending on goals both the design and permitted speed is applied in the planning of urban roads and streets. The design speed in Latvia is defined as the max possible driving speed (depending on stability on road and safety conditions) of a single vehicle at normal weather conditions and normal skid resistance of vehicle tyres on roadway surface. This speed is determining the design of geometrical parametres and, first of all, the choice of road cross- and longitudinal profile.

With the increase of the level of mobilisation high design speeds may not be efficiently implemented at extensive road loading. The world experience shows that a stabile trend to reduce the design speed in the road network developed already in 80-ies. It mostly concerns road design, but in urban areas it relates to the design of high-speed urban arterials.

The values for design speed specified in the temporary Latvian construction norms were taken over from the design norms of the Russian Federation used in 60-ies to 80-ies. At present they do not reflect the actual situation on roads and streets and do not allow the incorporation of urban environment limitations created by urban use of land and densely built-up areas. The experience of different countries shows that the values for design speed specified in the temporary Latvian construction norms for all street categories exceed the values specified in the European countries by 10 to 30 km/h.

The level of speed restrictions is determined in relation to the total min costs. However, considering that not all parametres may be expressed in monetary terms, the restrictions of permitted driving speed are chosen according to priorities. The priority for roads is the total number of road traffic accidents and their severity. For city arterial streets the priority is road capacity, and for local road network it is the safety of vulnerable road users.

If the permitted driving speed is considered from the point of view of max road capacity, then the lowest threshold of this speed has to be considered. However, if traffic safety is in question, the highest threshold of driving speed has to be considered. From the point of view of max road capacity the permitted speed on highways should be not less than 80 km/h but on urban arterial streets--not less than 60 km/h due to shorter distances between traffic nodes (Lazda, Smirnovs 2011).

In accordance with the European Transport Safety Council (ESTC) report in 1995 "Reducing Traffic Injuries Resulting Form Excess and Inappropriate Speed" road design speed may be defined as "the max speed that may be safely and comfortably maintained in the conditions of free traffic". This definition differs greatly from the past design speed definition used in Latvia which was based on the condition that vehicle was driving on the road solely but not in traffic flow.

In the essence the needed design speed depends on road function and thus on the desired driving level. If a road has several functions depending on the urban use of land, the lowest of the speed levels appropriate to each function has to be applied. At present the following limits of driving speed are applied in the European cities:

--70-80 km/h on roads with traffic flow function;

--50 km/h on roads with traffic distribution function;

--30 km/h on roads with access function.

However, if high speed is desirable because of the road function, the road quality and roadside safety furniture has to be in the appropriate level. Alternative for the improvement of road standard is the reduction of speed limits and the actual speed in accordance with the road standard and risks.

Undoubtedly the design speed may never be lower that the determined max permitted driving speed. On the other hand, it is also not wise to determine much lower max permitted driving speed than the design speed. This may discredit the reliability of max permitted driving speed.

In addition to that, it is important that design speed in sparsely built-up areas is consequent in as long road sections as possible. If the design speed is essentially reduced in any location the road profile has to be changed and appropriate road sings and markings have to be provided.

Driving speed, traffic flow capacity, etc. are the things that determine the desired level of traffic comfort and traffic safety. When planning the street network one has to understand clearly what the necessary or the desired speed on road should be. In accordance with ESTC, The JOINT OECD/ECMT and Danish Road Directorate report in 1991 "Urban Traffic Areas. Road Planning in Urban Areas" these desired speeds in urban environment could be classified in four main groups: low speed (0-20 km/h); medium low speed (30-40 km/h), medium high speed (50-60 km/h) and high speed (70-80 km/h).

When planning and designing roads one has to monitor consequently that the road users respect this desired speed (Hauer et al. 1982). When choosing the desired speed, road conditions, content of traffic flow, road user groups, road function aNd category have to be taken into account. The next step after defining the desired speed for each specific street is the choice of geometrical parametres depending on the design speed.

Therefore when planning and managing the traffic in urban street network the road owners have to be very careful in the choice of geometrical street parametres. In the essence they should be chosen not according to a specific road category as it was done in Latvia in the past years but according to real needs, namely, road function, traffic conditions and needs, financial conditions and also traffic safety requirements.

3. Results of the study

A practical study was carried out on 30 arterial streets with different applied design speeds in order to evaluate the substantiation of the chosen road profile and consequently street geometrical parametres in the existing street network. In random order the streets with different profiles (median, number of lanes, etc.) and different geometrical parametres (width of driving lanes, sidewalk width, etc.) were inspected and the appropriate [V.sub.design] and installed road signs for max permitted driving speed were registered. Table 4 shows the summary of roadway widths, lane numbers, lane widths, sidewalk widths, as well as, design speeds ([V.sub.design]) and max permitted driving speeds ([V.sub.perm]) of the inspected streets.

A problem when a street in the existing street network has high category but its function is low or traffic conditions is often encountered in Latvia. As the street category for city arterials is usually high, high [V.sub.design] was applied accordingly and automatically high geometrical parametres were implemented, as well. As the function of the street or traffic conditions are low, the road owner with respect to traffic organisation or traffic safety has been forced to decrease the desired driving speed on road. In the result unnecessarily expensive infrastructure has been created in the urban street network. Existence of good geometrical street parametres and inadequately low permitted max driving speed contributes to the violation of road traffic regulations thus creating unnecessary tension in the society and hindering traffic safety.

Practical study showed that the street function is incorrectly determined for most of the streets in the existing urban network and that the applied geometrical street dimensions are obviously inadequate. Depending on different traffic conditions the max permitted driving speed on arterial streets mostly is 50 km/h, but the streets themselves according to Table 2 were designed with [V.sub.design] = 70 km/h or even [V.sub.design] = 80 km/h which required inadequately high street geometrical parametres. Based on good practice examples for the planning of urban street network the author in Columns 8, 9 and 10 of Table 4 has made his proposals what could be the function of a specific street, what are the recommended design speed and max permitted driving speed, and what is the recommended width of driving lanes.

Thus inefficient and expensive infrastructurE is created that has negative impact on traffic safety because too wide driving lanes contribute to the exceeding of max permitted driving speed. The speed is more accurately estimated in peripheral sight, but underestimated in central sight. The studies revealed the impact of the perception of speed based on the available sphere of sight and they explain why drivers underestimate the speed on wide roads, because there are fewer points of reference. To conclude, the maintenance of such infrastructure also in future may not be feasible.

In addition to that, there are no grounds to specify design speed higher than 60 km/h for regulated city highways, as max permitted driving speed on regulated city highways may not exceed 50 km/h because traffic is regulated with traffic lights and in any case driving speed before traffic lights has to be reduced to 50 km/h.

4. Conclusions

1. It is obvious that the existing legislation on the design of street network in urban areas has to be revised with special attention paid to built-up areas. Appropriate [V.sub.design] has to be applied with respect to urban traffic conditions;

2. When drafting norms under Latvian conditions for the sections of urban streets, the good practice of developed countries should be taken as a basis for the standardisation of street functions with special emphasis of self-explaining roads. It is proposed to use the following main road functions in the planning of urban road network: traffic flow, traffic distribution and access functions, with contribution to simplified understanding and proper use of these functions;

3. Together with the revision of norms the road owners should review the actual functions of streets with respect to traffic conditions in the whole length of the street, as well as, the desired driving speed. Only after that road engineers should choose respective geometrical parametres in their plans and designs;

4. All the above mentioned should serve as pre-condition for the creation of such road infrastructure where drivers acquire knowledge on road environment based on their experience and they themselves organise roads into categories, wherewith a number of additional traffic organisation measures, monitoring and control of traffic flow would become unnecessary.

doi: 10.3846/bjrbe.2011.36

Received 20 April 2010; accepted 4 November 2011

References

Antov, D.; Abel, K.; Surje, P.; Rouk, H.; Roivas, T. 2009. Speed Reduction Effects of Urban Roundabouts, The Baltic Journal of Road and Bridge Engineering 4(1): 22-26. doi:10.3846/1822-427X.2009.4.22-26

Cameron, M. H.; Elvik, R. 2010. Nilsson's Power Model Connecting Speed and Road Trauma: Applicability by Road Type and Alternative Models for Urban Roads, Accident Analysis & Prevention 42(6): 1908-1915. doi:10.1016/j.aap.2010.05.012

Hauer, E.; Ahlin, F. J.; Bowser, J. S. 1982. Speed Enforcement and Speed Choice, Accident Analysis & Prevention 14(4): 267278. doi:10.1016/0001-4575(82)90038-0

Ratkeviciute, K.; Cygas, D.; Laurinavicius, A.; Maciulis, A. 2007. Analysis and Evaluation of the Efficiency of Road Safety Measures Applied to Lithuanian Roads, The Baltic Journal of Road and Bridge Engineering 2(2): 81-87.

Lama, A.; Smirnovs, J.; Naud[]uns, J. 2006. Road Traffic Safety in the Baltic States, The Baltic Journal of Road and Bridge Engineering 1 (1): 63-68.

Lazda, Z.; Smirnovs, J. 2009. Evaluation of Road Traffic Safety Level in the State Main Road Network of Latvia, The Baltic Journal of Road and Bridge Engineering 4(4): 156-160. doi:10.3846/1822-427X.2009. 4.156-160

Lazda, Z.; Smirnovs, J. 2011. Research on Traffic Flow Speed of Arterial Streets in Urban Areas, in Proc. of the 8th International Conference "Environmental Engineering": selected papers, vol. 3. Ed. by Cygas, D.; Froehner, K. D. May 19-20, 2011, Vilnius, Lithuania. Vilnius: Technika, 936-941.

Zilioniene, D.; Oginskas, R.; Petkevicius, K. 2010. Research, Analysis and Evaluation of Roundabouts Constructed in Lithuania, The Baltic Journal of Road and Bridge Engineering 5 (4): 240-245. doi:10.3846/bjrbe.2010.32

Ziedonis Lazda (1) Juris Smirnovs (2)

Institute of Transport Structures, Civil Engineering Faculty, Riga Technical University, Azenes 16, LV 1048 Riga, Latvia

E-mails: (1) ziedonis.lazda@csdd.gov.lv; (2) smirnovs@bf.rtu.lv

Table 1. Street categories and their basic importance specified
in the norms (LBN 100; ChuII 1989)

Street category             Street importance

Highways (arterial          --high-speed transport
  roads) *:                 connections within urban
                            territory;

--high-speed (crossings     --exits to main roads outside urban areas,
only  in separate grades)   to airports, recreation areas, other urban
                            areas.

--regulated flow            --transport connections between urban
(crossings  mostly at       districts in specific directions and
grade)                      sections outside residential areas that
                            have intensive, mostly freight traffic;

                            --exits to main roads outside urban areas.

City arterials (arterial
  streets) *:

1. Urban importance
(general urban
importance) *

--free flow (crossings in   --transport connections between urban
main  directions only in    districts and centres, as well as, to
separate grades)            other arterials and roads outside urban
                            areas;

--regulated flow            --transport connections between urban
(crossings with  highways   districts and centres;
and arterials mostly at
grade)
                            --exits to other roads and arterials and
                            roads outside urban areas.

2. District importance

--transport and             --transport and pedestrian connections
pedestrian                  between urban territories and public
                            centres;

                            --exits to other arterials.

--pedestrian and            --pedestrian and transport (mostly public
transport                   transport) connections within separate
                            urban territories.

Streets of local
  importance  (local
  streets and roads): *

1. Residential streets;     --pedestrian and transport (except freight
                            and public transport) flow within
                            residential territories;

                            --exits to regulated roads and arterials.

2. Freight transport        --mostly freight and car transport within
  streets;                  industrial areas;

                            --exits to urban roads.

3. Pedestrian streets and   --pedestrian traffic to workplaces and
roads;                      recreation areas, service centres, public
                            transport stops, including public centres.

4. Park roads;              --pedestrian and transport traffic in
                            parks and forest territories.

5. Access roads;            --transport accesses to residential and
                            public buildings and their groups and
                            other buildings within city blocks.

6. Cycling roads            --cycling on tracks where no other
                            vehicles are allowed.

*--differences between the previously mentioned sources of reference
are given

Table 2. Calculation parameters for urban streets specified in norms
(LBN 100; ChuII 1989)

Street category                      Design speed,   Driving lane
                                          km/h          width, m

Highways (arterial roads) * :
--express                                 120            3.75
--regulated traffic                       80             3.50

Arterials (arterial streets) *:
Urban (general urban importance) *
--free flow                               100            3.75
--regulated flow                          80             3.50
District importance
--vehicle and pedestrian                  70             3.50
--pedestrian and vehicle                  50             4.00

Local importance (local roads and
 streets): *

--residential streets                     40             3.00
                                          30             3.00
--freight transport streets               50             3.50
                                          40             3.50
--park roads                              40             3.00

Access roads
--main                                    40             2.75
--secondary                               30             3.50

Pedestrian streets
--main                                    --             1.00
--secondary                                -             0.75

Cycling roads
--separated                               20             1.50
--isolated                                30             1.50

                                                       Min
Street category                       Number of     horizontal
                                         lanes      curvature
                                                    radius, m

Highways (arterial roads) * :
--express                                4-8           600
--regulated traffic                      2-6           400

Arterials (arterial streets) *:
Urban (general urban importance) *
--free flow                              4-8           500
--regulated flow                         4-8           400
District importance
--vehicle and pedestrian                 2-4           250
--pedestrian and vehicle                  4            125

Local importance (local roads and
 streets): *

--residential streets                   2-3 **          90
                                          2             50
--freight transport streets              2-4            90
                                          2             50
--park roads                              2             75

Access roads
--main                                    2             50
--secondary                               1             25

Pedestrian streets
--main                               according to       --
--secondary                           calculat.         --

Cycling roads
--separated                              1-2            30
--isolated                               2-4            50

Street category                              Max
                                         longitudinal
                                     fall, [per thousand]

Highways (arterial roads) * :
--express                                     30
--regulated traffic                           50

Arterials (arterial streets) *:
Urban (general urban importance) *
--free flow                                   40
--regulated flow                              50
District importance
--vehicle and pedestrian                      60
--pedestrian and vehicle                      40

Local importance (local roads and
 streets): *

--residential streets                         70
                                              80
--freight transport streets                   60
                                              70
--park roads                                  80

Access roads
--main                                        70
--secondary                                   80

Pedestrian streets
--main                                        40
--secondary                                   60

Cycling roads
--separated                                   40
--isolated                                    30

Street category                        Sidewalk
                                         width,

Highways (arterial roads) * :
--express                                 --
--regulated traffic                       --

Arterials (arterial streets) *:
Urban (general urban importance) *
--free flow                              4.5
--regulated flow                         3.0
District importance
--vehicle and pedestrian
--pedestrian and vehicle                 2.25
                                         3.0
Local importance (local roads and
 streets): *

--residential streets                    1.5
                                         1.5
--freight transport streets              1.5
                                         1.5
--park roads                              --

Access roads
--main                                   1.0
--secondary                              0.75

Pedestrian streets
--main                               according to
--secondary                             design

Cycling roads
--separated                               --
--isolated                                --

*--differences between the previously mentioned sources of reference
are given

**--one lane is used for car parking

Table 3. Road categories and road functions according The JOINT
OECD/ECMT report in 2006

ENvironment           Road category              Road function

Outside urban areas   Motorways (inter-urban)    Flow
                      Main roads                 Flow
                      Motorways (inter-urban     Flow
                        main roads)
                      Main roads outside urban   Flow/distribution
                        areas
                      Secondary roads outside    Access
                        urban areas

In urban areas        City arterials (urban)     Flow
                      City arterials and main    Flow/distribution
                        roads
                      Urban roads                Access

Table 4. Geometrical parametres of streets in urban street network

Street   Carriageway   Number of   Lane       Sidewalk
No.      width, m      lanes       width, m   width, m

1        11.03          2            5.51      2.94
2        11.08          2            5.54      2.92
3         9.18          2            3.77      --
4         9.20          2            3.69      --
5        14.03          2            6.96      2.50
6         9.60          2            4.75      --
7        14.10          4            3.40      2.0
8        11.50          2            5.70      1.53
9        14.43          2            7.21      1.52
10        9.15          2            4.52      1.95
28       10.90          2            4.55      --
29        8.92          2            4.46      2.15
30       11.10          2            4.70      --

Street   [V.sub.perm']   [V.sub.design']
No.

1        50               80
2        50               70
3        70              100
4        50              100
5        50               80
6        50               80
7        70               80
8        50               70
9        50               70
10       50               80
28       90              100
29       50               70
30       70              100

                        Recommended
Street   (actual)       [V.sub. design]   land width, m
No.      function       ([V.sub.perm]),
                        km/h

1        distribution   60(50)            3.00-3.25
2        distribution   60(50)            3.00-3.25
3        flow           80(70)            3.5
4        flow           80(70)            3.5
5        distribution   60(50)            3.00-3.25
6        flow           80(70)            3.5
7        flow           80(70)            3.5
8        distribution   60(50)            3.00-3.25
9        distribution   60(50)            3.00-3.25
10       flow           80(70)            3.5
28       flow           90(80)            3.5
29       distribution   60(50)            3.00-3.25
30       flow           90(80)            3.5