Principles for safe road design

The road network in the Netherlands is constructed according to the principle of functional network structure. This is based on two main traffic functions of roads: flow and access (residence). In the first case, the motorized traffic (usually over longer distances) travels under fast, efficient and safe conditions. In the second case, the road provides access to homes, stores, offices, companies etc. for all types of traffic. A third (additional) function for a road is connecting roads with a flow function and roads with an access function. It is of great importance to road safety that each road has only one function: mono-functionality.

Based on the functional network structure and the Sustainable Safety vision (see also SWOV-factsheet Sustainably Road Safety) the Netherlands has three main road categories (see also the question How is the road network in the Netherlands structured?). Each road category has its own design principles and characteristics:

1. Through roads allow traffic to travel from origin to destination as quickly and safely as possible ('flow'). Car traffic has the highest priority. Through roads may only be situated outside urban areas. We then refer to trunk roads and motorways.

2. Access roads offer direct access to residential areas at the locations of origin and destination. The residential function is most important and the car traffic needs to adapt to the 'residents' such as cyclists and pedestrians (in particular by traveling at low speeds). Access roads can be found in urban areas and in rural areas. These are 30 km/h roads and homezones (15 km/h) in urban areas and 60 km/h roads in rural areas.

3. Distributor roads connect the through roads with the access roads. The traffic flows at road sections and exchange occurs at intersections. Distributor roads come are found in both urban and rural areas. These roads are mainly the 50-and 70 km/h roads in urban areas and the 80 km/h roads in rural areas.

The categorization of roads is helpful for road authorities as well as for road users.

Road authorities

Categorization of roads allows road authorities to ensure an efficient traffic flow in their management areas: through traffic between two residential areas, for example, needs to be directed to a through road as quickly as possible. Furthermore, the categorization of roads provides handles for the design and layout of the roads.

Road users

In principle categorization of roads is also helpful for the road users. However, it is essential that roads with different functions have a consistent and recognizable layout. This way road users will know what behaviour is expected of them, which other road users they can encounter and how these in all likelihood will behave (predictability).

The road design is in fact based on the triangle Function – Design – Usage which is also used by designers outside the 'traffic' work field.

This triangle indicates that the function or purpose of the design should be reflected by the road layout and that the road design should evoke the intended and desired behaviour and road usage. When applied to traffic, this means that there is an optimal road design for each of the three traffic functions (flow, access and connect). This results in the intended and desired use by road users. From the road user’s point of view the road layout should make clear the intended function and (consequently) the desired behaviour. This is called predictability.

The actual implementation of the design guidelines is based on the Sustainable Safety concept and the Sustainable Safety-principles of functionality, homogeneity and predictability (see also SWOV Fact sheet Sustainably Road Safety).

The Sustainable Safety vision has led to twelve general functional requirements for a safe road network, safe routes and a safe road design [2].

At the road network level:

1. Make residential areas as large as possible and contiguous.
2. Make sure that the amount of travel on relatively unsafe roads is as small as possible.
3. Make journeys as short as possible.
4. Make sure that the concepts 'shortest ride' and 'safest ride' coincide.

The requirements at road network level ensure that cars which drive in residential areas almost exclusively have their destination or origin in that area. Pedestrians and cyclists in residential areas therefore encounter less motorized traffic; this increases safety [3]. Exposure to hazards and to traffic in general diminishes when requirements 2, 3 and 4 are complied with.

At the level of routes:

5. Prevent search behaviour.
6. Make categories as predictable as possible.
7. Limit the number of safety solutions and make them more uniform.

By these requirements at the route level traffic can use the roads in a way for which they have been designed as much as possible. The design (of a road category) is appropriate to the intended use; the design especially increases safety if the actual use does not differ too much from the intended use [1].

At the level of road design:

8. Prevent conflicts with oncoming traffic.
9. Prevent conflicts with crossing and traversing traffic.
10. Separate traffic types.
11. Reduce the speed at potential conflict points.
12. Prevent obstacles at the side of the road.

The requirements of the road design are intended to prevent crashes and to reduce the severity of the consequences of unavoidable crashes. These requirements as much as possible prevent the possible conflicts between vehicles, for example by separating driving directions provided or by lowering speeds at intersections [4].

Through roads, i.e. trunk roads and motorways, are exclusively intended for high-speed travel from A to B (flow function) and are only accessible for fast traffic.

Motorways consist of at least 2 x 2 lanes and driving directions are always physically separated, generally by a central median and/or guide rail. At the right-hand side through roads mostly have a wide emergency lane and a wide obstacle-free zone. The general limit is 130 km/h, but at certain locations or times the limit is 80, 100 or 120 km/h. These lower limits are applied because of safety, flow or environment.

Generally, the layout of trunk roads is less safe than that of motorways (e.g. narrower lanes, a smaller obstacle-free zone and a less wide/hard physical separation between the driving directions). The speed limit on trunk roads is usually 100 km/h.

Intersections with and between motorways are always grade separated. In principle, intersections between trunk roads are also grade separated, but in practice this is not (yet) always the case. Intersections between trunk roads and distributor roads are often controlled with a roundabout or with traffic lights.  

More information on the design of through roads can be found in the CROW publications on this topic [5] [6]. These publications are in the Dutch language.

Through road - Motorway

Through road - Trunk road

Access roads provide access to homes, businesses, schools, shops, et cetera. Access roads can be found in areas with a residential function. This means that all types of traffic mix here: pedestrians, cyclists, cars, and trucks. Because of the great difference in mass between the road users and the fact that pedestrians and cyclists are largely unprotected, the speed of motorized traffic on access roads must be low.

Access road in an urban area (Photograph: Paul Voorham)

Urban areas

Access roads in urban areas have a 30 km/h speed limit. Enforcement of this limit usually requires physical speed inhibitors. For more information, see SWOV Fact sheet 30 km/h zones. In addition to 30km/h access roads there are homezones. Homezones have a 15 km/h speed limit and pedestrians can walk and play on the entire width of the street. Homezones can be found not only in residential areas, but also in shopping areas and in the vicinity of train stations.

Rural areas

The speed limit on rural access roads the limit is 60 km/h. This is a compromise between requirements for traffic flow on the one hand and requirements for safety on the other hand. Generally, the physical speed inhibitors are only used at intersections (raised junction). Road sections are often fitted with edge strips (a broken line at some distance from the roadsides, sometimes asphalted in red), whereby a driving strip for motor vehicles is created in the middle of the roadway. This leads to a visual narrowing of the road which results in lower speeds. With sufficient width the side strips on either side of the driving strip can be used by cyclists. For more information, see the archived SWOV Fact sheet Shoulders on rural access roads.

Rural access road (Photograph: Paul Voorham)

Intersections

• Intersections between access roads are at grade and without any designated priorities (priority for traffic from the right).
• Intersections between an access road and a distributor road are also at grade. Traffic on the distributor road has priority and the connection from the access road has a so-called exit construction.

Access road ending in an exit construction (Photograph: Paul Voorham)

More information on the design of access roads can be found in the CROW publications on this topic [7] [5]. These publications are in the Dutch language.

Distributor roads connect roads with a residential function (access roads) and roads with a flow function (through roads). The road sections of a distributor road have a flow function. The speeds of motorized traffic are relatively high and therefore cyclists should preferably have their own separate facilities (see also SWOV Fact sheet Infrastructure for pedestrians and cyclists).

At the intersections exchange takes place. This is where motorized traffic and pedestrians and cyclists encounter each other and the speed of motorized traffic must be low. This can for instance be achieved by roundabouts. In practice, many distributor roads (especially in urban areas) also have an access function, for example distributor roads with many shops. Such roads are also called grey roads [8].

Urban areas

Urban distributor roads usually have a 50 km/h speed limit, sometimes a 70 km/h limit. There are separate facilities for cyclists and light moped riders. Preferably this is a physically separated cycle path. Less safe, but also often applied, is a cycle lane, separated from the lanes for motorized traffic by a broken or solid line. On road sections with a 50 km/h speed limit the moped rider generally uses the roadway.

Rural areas

Distributor roads outside urban areas the generally have an 80 km/h speed limit. On the road sections the bicycle facilities are always physically separated. The moped must use the cycle/moped path here. Because of the great speed differences agricultural traffic generally does not use the lanes of a distributor road, nor does it use the cycle/moped path due to the large mass difference. However, this can often not be realized for lack of parallel roads or realistic alternative routes. Preferably, the driving directions are physically separated by a median or by flaps, or else by a double solid line.

Intersections

• In principle, intersections between two distributor roads are basically grade separated and are preferably regulated with a roundabout (see SWOV Fact sheet Roundabouts). Roundabouts cannot be applied if the traffic flows are too large. Then traffic signals are necessary. Sometimes plateaus are placed just in front of the traffic-signal-regulated intersection to reduce speeds at the intersection to about 50 km/h [9] .
• At intersections between a distributor road and an access road the traffic on the distributor road has priority and the access road has an exit construction.

In principle, an intersection between a distributor road and a through road is grade separated. It is important that the entry ramp to a motorway can clearly be distinguished from the motorway exit ramp, to prevent wrong-way driving (see also SWOV Fact sheet Wrong-way driving). Sometimes an at grade intersection is used, mostly traffic signal regulated.

More information on the design of distributor roads can be found in the CROW publications on this topic [7] [5]. These publications are in the Dutch language.

Road sections

If motorized traffic should be able to flow and travel at high speed, such as on the road sections of distributor roads, there must be a physical separation between the motorized traffic and cyclists. This can best be achieved by separate cycle paths. In the construction of bicycle facilities it is important to ensure that no conflicts may arise between parking vehicles (manoeuvers, car doors being opened and disembarking passengers) and passing cyclists. Crashes occur relatively more frequently on a two-way cycle path than on a one-way cycle path [10]. The width of the cycle path, the quality of the road surface, and the presence of bollards and other obstacles are other factors that influence the safety of bicycle facilities (see also SWOV Fact sheet Cyclists).

Intersections

At intersections cyclists mix with car traffic and travel speeds of motorized traffic should therefore be lower than on the connecting road sections. When roundabouts have been applied the speeds driven are automatically quite low (see also SWOV Fact sheet Roundabouts). At other types of intersections a lower speed must be enforced by speed-reducing measures such as speed humps just before the intersection or a raised intersection (plateau). At intersections raised crossing facilities for cyclists are the best option. At intersections with busy roads a traffic island makes it safer for cyclists to cross the road.

The CROW publication Design manual for bicycle traffic [11] provides more information on this topic.

In the Netherlands, the predictability of roads is mainly executed by using markings that differ per road category. The figure below shows the markings that are used. These markings have been applied on most roads. Making roads predictable ensures that road users always know what kind of road they are driving on, who they may encounter there and how to behave. For the predictability of roads it is not only important that there is a distinction is between categories, but that there also needs to be uniformity within categories. Median markings and separations of driving direction are features that are meaningful for the road user. In addition, the design of intersections at transitions can be helpful in making road types recognizable. Public service advertising can improve the knowledge about the meaning of markings, but in practice the driving behaviour has been found to be mainly intuitively evoked by the road design [12]. In 2004, CROW has published guidelines for the essential predictability features of the road infrastructure [13].At some stage vehicles will be equipped with electronics that make use of the essential predictability features. Then it is necessary that these essential predictability features are consistently applied everywhere [14].

More information can be found in the archived SWOV Fact sheet Recognizable road design.

Table 1. Essential predictability features further developed [15].Each road type has its own zone indication; the markings have been developed per road type and can be applied in both urban and rural areas. The physical separation of driving direction applies to through roads as well as to both regional (SW100) and national through roads (SW120). In accordance with the guidelines , the green centre marking may also be used for regional through roads.

There are several instruments that road authorities can (commission to) use to test the safety of their road network and their roads. This provides an overview of possible bottlenecks in the safety and (consequently) of the targets for action.

Planning and designing tests

The safety of a new road or a far-reaching redesign of a road can be tested in advance by carrying out a road safety audit in various phases of the planning and of the design (see also archived SWOV Fact sheet The Road Safety Audit and Road Safety Inspection).

Testing existing roads

Various tests can be used to test the safety of existing road infrastructure. The table below presents an overview. The first two instruments focus on the main road network only. The other instruments (also) address the underlying road network; some of the tests (DV-meter, VSGS) focus on the design and layout of roads and others (route test, cores method) mainly focus on the road network. ProMeV combines elements of different instruments.

Table 2. Overview of instruments to test the safety of road infrastructure. Based on: Korving et al., 2016 [17].

Testing the safety of bicycle infrastructure

The tool CycleRAP is being developed specifically for the bicycle infrastructure[18] [19]. At present he tool has three safety indicators:

1. The general quality of the bicycle infrastructure (characteristics such as width of bicycle facility, holes/bumps in surface or verge, high curb adjoining bicycle facility).
2. The presence of obstacles (characteristics such as bollards or traffic islands on the bicycle facility).
3. General length-height profile (features such as sharp bend, narrowing, slope, marking, lighting).

Guidelines for road design and road layout are published by CROW, the knowledge platform for infrastructure, traffic, transport and public space in the Netherlands (www.crow.nl). These guidelines are made in so-called CROW working groups which are formed by traffic and road safety experts from various local authorities, consultancy firms and knowledge institutes. The guidelines are often a compromise between accessibility, environment and safety. Where possible, guidelines are substantiated by scientific knowledge, but as yet not sufficient scientific knowledge is available for all aspects. This means experiences in practice are also important in drawing up guidelines.

In the Netherlands the guidelines for road design are not legally binding. A road authority should, however, have good reasons to deviate from the guidelines as it is responsible for the quality of its roads [20]. In the event of a crash on a road where the guidelines have not been properly applied, the road authority may be held liable. In a possible lawsuit, the road authority must be able to motivate why the guidelines were not complied with. He must also prove that the chosen alternative is at least equally safe.

The extent to which the guidelines are applied varies greatly between road authorities and between different projects. The application of guidelines depends on the availability of space and with the combination of traffic and environmental features of a road: application can be hampered by lack of space, but also because the function and use of a road differ too much. Sometimes there are also substantive reasons to depart from the guidelines: there are different road safety interests which contradict each other. An example is the question whether a two-way cycle path along a ring road should be constructed at the city centre side or on the outside. Both solutions have unsafe traffic consequences. In this case the choice is generally customization, with thorough investigation of which compensatory measures are possible. These are often speed-reducing measures. In addition, a traffic design is subject to various constraints: regional integration, political choices, interests of those directly involved. Also in the design phase choices must be made that will affect the end result. It is important that road safety aspects play a proper role in these design choices. The Dutch guidelines only give limited clarity about the knowledge on which they are based. Only in just over 30% of the design characteristics a road safety effect on is mentioned [21]. At this moment it is therefore almost impossible to determine the (quantitative) road safety effect when the guidelines must be or by choice are deviated from. Therefore a qualitative assessment by an expert plays an important role. Such an assessment can possibly provide more clarity, but gives no 'hard' results [22] [23].

For vehicles equipped with electronics that respond to the presence of certain road features, all relevant road features should consistently be present at all locations. Given the previously mentioned shortcomings in design and actual implementation, this is difficult to achieve.

Below you will find the list of references that are used in this fact sheet. Our knowledge portal offers more literature on this subject.

[1]. Wegman, F. & Aarts, L. (2006). dvancing Sustainable Safety; National Road Safety Outlook for 2005-2020. . SWOV, Leidschendam.

[2]. CROW (1997). Handboek categorisering wegen op duurzaam veilige basis. Deel 1: (voorlopige) functionele en operationele eisen. Publicatie 116. CROW, Ede.

[3]. Dijkstra, A. (2011). En route to safer roads: How road structure and road classification can affect road safety. Proefschrift Universiteit Twente, SWOV Dissertatiereeks. SWOV, Leidschendam.

[4]. Dijkstra, A. (2003). Infrastructurele verkeersvoorzieningen en hun veiligheidsaspecten; De betekenis van de verschillende soorten verkeersvoorzieningen voor een duurzaam-veilig verkeers- en vervoerssysteem. D-2003-5. SWOV, Leidschendam.

[5]. CROW (2013). Handboek wegontwerp 2013; Regionale stroomwegen. CROW, Ede.

[6]. CROW (2014). Richtlijn Ontwerp Autosnelwegen. CROW, Ede.

[7]. CROW (2012). ASVV Aanbevelingen voor stedelijke verkeersvoorzieningen. CROW, Ede.

[8]. Dijkstra, A., Eenink, R.G. & Wegman, F.C.M. (2007). Met een veilige snelheid over wegen; SWOV-visie op 'de grijze weg'. In: Verkeerskunde, vol. 58, nr. 7, p. 48-52.

[9]. Fortuijn, L.G.H., Carton, P.J. & Fedds, B.J. (2005). Veiligheidseffecten van kruispuntplateaus in gebiedsontsluitingswegen. Verkeerskundige Werkdagen 2005. CROW, Ede.

[10]. Schepers, P. (2013). A safer road environment for cyclists. Proefschrift Technische Universiteit Delft, Leidschendam.

[11]. CROW (2016). Design manual for bicycle traffic.. Record no. 28. CROW, Ede.

[12]. Aarts, L.T., Davidse, R.J. & Christoph, M. (2007). Herkenbaar wegontwerp en rijgedrag. Een rijsimulatorstudie naar herkenbaarheid van gebiedsontsluitingswegen buiten de bebouwde kom. R-2006-17. SWOV, Leidschendam.

[13]. CROW (2004). Richtlijn Essentiële Herkenbaarheidkenmerken van weginfrastructuur. Publicatie 203. CROW, Ede.

[14]. EuroRAP (2013). Roads that cars can read. A quality standard for road markings and traffic signs on major rural roads.

[15]. CROW (2012). Basiskenmerken wegontwerp: categorisering en inrichting van wegen. Publicatie 315. CROW, Ede.

[16]. Schepers, P. & Janssen-Stans, Y. (2017). VeiligheidsINDicator 1.0. Rijkswaterstaat, Dienst Water, Verkeer en Leefomgeving, Utrecht.

[17]. Korving, H., Goldenbeld, C., Schagen, I. van, Weijermars, W., et al. (2016). Monitor Beleidsimpuls Verkeersveiligheid 2016 – Achtergrondinformatie en onderzoeksverantwoording. R-2016-14A. SWOV, Den Haag.

[18]. Wijlhuizen, G.J., Dijkstra, A. & Petegem, J.H. van (2014). Safe Cycling Network. Ontwikkeling van een systeem ter beoordeling van de veiligheid van fietsinfrastructuur. R-2014-14. SWOV, Den Haag.

[19]. Wijlhuizen, G.J., Petegem, J.W.H. van, Goldenbeld, C., Gent, P. van, et al. (2016). Doorontwikkeling CycleRAP-instrument voor veiligheidsbeoordeling fietsinfrastructuur. R-2016-11. SWOV, Den Haag.

[20]. Snoeren, P.W.M. (2008). Een bon voor de wegbeheerder? Vrijheid in vormgeving en inrichting van wegen aan banden. P.W.M. Snoeren, Nieuwegein.

[21]. Bax, C.A. (2011). Processes and patterns; The utilisation of knowledge in Dutch road safety policy. Proefschrift Radboud Universiteit Nijmegen, SWOV-Dissertatiereeks. SWOV, Leidschendam.

[22]. Bax, C., Petegem, J.H. van & Giesen, M. (2014). Passen gemeenten de Ontwerpwijzer Fietsverkeer toe? Gebruik van de richtlijnen voor fietsinfrastructuur en factoren die dit beïnvloeden. R-2014-23. SWOV, Den Haag.

[23]. Schermers, G., Dijkstra, A., Mesken, J. & Baan, D. de (2013). Richtlijnen voor wegontwerp tegen het licht gehouden; de mate van onderbouwing van bestaande richtlijnen voor het ontwerp van gebiedsontsluitingswegen binnen en buiten de bebouwde kom en van stroomwegen. D-2013-5. SWOV, Leidschendam.