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Road Safety Engineering for Speed Bumps and Raised Crossings

Road Safety Engineering for Speed Bumps and Raised Crossings[1]

 

Speed bumps designed for passing speeds of 20, 30, 50, and 60 km/h with heights of 8 or 12 cm all work effectively as traffic calming measures aimed at speed reduction.

This also applies to raised crossings at 30, 50, and 60 km/h roads with a raised table (platform) fat the same heights. A speed bump or platform with a height of 8 cm is used when a 12 cm height causes clearance issues due to the low ground clearance of large vehicles, for example, public buses with 'low floors' or agricultural transport on rural roads. The choice of height is up to the road manager. This guideline applies to speed bumps and raised crossings in both urban and rural areas. When a guideline has a specific area of application, this is indicated.

Uniformity in dimensions is highly desirable. Generally, raised crossings are perceived more positively than speed bumps, as the application of raised crossings at intersections and slow-traffic junctions is better understood and therefore more accepted. A downside is the variation in design and structure of raised crossings. Applying this guideline promotes uniformity in vertical speed controls.

Figure 1: Speed Bump on a Main Road (Netherlands)

Speed bumps and raised crossings with passing speeds of (20), 30, 50, and 60 km/h have a height of 8 or 12 cm and are constructed across the full width of the road. The sinusoidal shape appears most suitable for speed bumps and raised crossings, and the installation technique is the most decisive factor for the height of the speed bump. The table below shows recommended speed bump dimensions. Speed bumps with passing speeds of 20, 30, and 50 km/h and a height of 12 cm are suitable as speed controls. Details are provided in the table below and in Figure 4: Speed Humps standard details (Source: CROW) at the end of this article.

Speed bumps with a height of 8 cm complement this setup and can be used where a height of 12 cm poses clearance problems for large vehicles. All speed bumps except one are sinusoidal. The sinusoidal shape is challenging to achieve on longer slopes, hence a trapezoidal shape may be selected.

The sinusoidal shape appears most suitable. The idea is that this shape can be passed without too much discomfort at the advised speed. Exceeding this speed therefore causes nuisance and encourages traffic to reduce their driving speed to the advised speed (which is the V85 speed). This is contrary to many traditional speed humps that force drivers to speeds considerably lower than the V85 whit the result that they pull up again to higher speeds after passing the speed hump.

Table 1: Dimensions of Full-Width Speed Bumps

Passing Speed V85 [km/h]

Profile

Height [m]

Ascending Ramp length [m]

Level Length [m]

Descending Ramp length [m]

Total length [m]

20

Sinus

0.12

1.70

-

1.70

3.40

20

Sinus

0.08

1.00

-

1.00

2.00

30

Sinus

0.12

2.40

-

2.40

4.80

30

Sinus

0.08

1.75

-

1.75

3.50

50

Trapezium

0.12

4.80

2.40

4.80

12.00

50

Sinus

0.08

3.00

-

3.00

6.00

60

Sinus

0.12

6.00

-

6.00

12.00

60

Sinus

0.08

4.00

-

4.00

8.00

 

Points to Consider When Applying Speed Bumps and Raised Crossings

  1. The following application rules are maintained[2]:
    1. Not appropriate for roads with 85th-percentile speeds of 70 km/hr or more
    2. Not recommended on grades greater than 8 percent
    3. Work well in combination with curb extensions in urban areas
    4. Can be used on a one-lane one-way or two-lane two-way street
  2. In cases where the approach speed V85 is high (80 km/h or more), additional measures (rumble strips, signage) are strongly recommended to bring down the approach speed below 70 km/hr.
  3. Especially in the period immediately after speed bump installation, communication with traffic via warning signs will be essential to prevent high-speed passages and potential loss of vehicle control or vehicle damage.
  4. When speed bumps or raised crossings as traffic calming measures are introduced for the first time in an area where drivers are generally not used to these types of traffic calming measures, a publication campaign via media (also social media) may be advisable.

Figure 2: Speed Bump on an Access Road (Netherlands)

  1. In the case of a residential area with a relatively high volume of freight traffic (number of heavy duty vehicles in the traffic flow above 5%) the area is probably not a residential area, and this measure is not recommended; instead, consider an alternative, such as a chicane (Figure 3). Also, the question arises why this heavy duty traffic is passing through the area (better be diverted).
  2. What is the V85 speed on the road? This is the operational speed that is not exceeded by 85% of the traffic on the specific road section as measured over a certain period of time. This speed must not exceed the target passing speed (or design speed) by more than 25 km/h. If the difference is greater, the speed bump is unsafe, and other traffic calming (speed reduction) measures are advised.
  3. Are there low-clearance vehicles on the road? If so, an 8 cm bump/plateau is preferable over a 12 cm height.
  4. In case speed humps are applied to DBST paved roads, the structural stability ca be doubted. The hump is exposed to a horizontal force of the wheels of the passing vehicle and the DBST pavement could be damaged. In DBST paved roads the humps would better be constructed of more sturdy pavement material.


Figure 3: Chicane (Canada)

Figure 4: Speed Bumps standard details (Source:CROW)

[1] Publications of the  Centre for Rational Research in Roads and Bridges (CROW, Netherlands) have been used as source material with complements to fit into an international context

[2] Source: ITE (Community of Transportation Professionals, USA)

Road Safety Awareness Campaign Objectives with Climate Change Resilient Improvement

Each road safety awareness campaign may have its specific objectives, based on the composition of the populace in the project area and the presence of specific target groups Great differences may be expected between urban and rural areas. Awareness campaigns had the following four specific objectives may be considered:

  • Increase awareness of road users living near the project roads on safe road use behavior especially at junctions and road crossings

  • Inform and educate road users in the target communities of the new road scheme to enhance safe road use

  • Incorporate gender equality, disability and social inclusion -specific aspects of road safety

  • Increase public awareness of the dignity and rights of persons with disabilities in regard of road safety

  • Create awareness of the need to protect investments in road infrastructure from climate change

  • Provide information on road signs and pavement markings

  • Identify road safety issues/concerns in the street communities, of specific target groups 

  • Roleplay at school during awareness campaign

  • Road safety awareness team conveying the message to tuktuk drivers

    This list is of course not exhaustive. The road safety awareness campaign is sometimes combined with aspects related to;

    • Climate change resilience

    • Environmental & Social safeguard issues

    • HIV/AIDS and other sexually transmittable diseases

    • Gender issues and the  prevention of gender-based violence and other gender issues at the community level

    Such aspects can become pre-dominant in the campaign when specific target groups are included, such as contractor’s labourers, commercial vehicle drivers, indigenous people. Awareness raising on gender, core labour standards or labour management procedures with regard to correct road user behaviour is then included in the campaign.

    • Climate resilient roads

    • Climate resilient roads

      Baseline Awareness Survey

      The baseline study is conducted to collect data on knowledge, attitude and self-reported behaviour on road safety of specific road users. These so-called target groups can be adult road users (both motorized and non-motorized) and school children (primary and secondary schools).


      Data are collected using standardized questionnaires administered through face-to-face interviews for each of the identified target groups. Being a longitudinal study the adult road users are selected through the support of local leaders from among residents of the communities adjacent to the project road (in case of single road link study) or living within the project area (in case of a road network study). For selection of schools the wider area along the road link or the road project area should be considered. In urban but moreover in rural areas school children walk or cycle long distances to and from school. Motorized road users for awareness surveys can be further distinguished for instance: (long distance) lorry drivers, motorcyclists, bus and taxi drivers.


      Equal participation of both male and female road users should be planned in the study for residents and school children. Where commercial activities are predominant, commuters should be included in the group of residents.


      In case of specific motorized road users an equal participation by gender would be preferred but might be difficult to achieve in practice due to culture/tradition. Representation by education and occupation should reflect the composition of the residential/commuters’ populace in the project area. It will be clear that preparation of the baseline survey requires extensive efforts.


      Subjects to be investigated during the baseline survey are:

      • Road safety concerns

      • Knowledge on road safety

      • Attitudes, opinions, perceptions on road safety issues

      • Behavior patterns

      • Baseline survey data collection Tanzania

      • Road safety data collectiion through face to face interviews with bus drivers

      • Interviewing during site visit as part of reconnaissance survey

        Awareness Campaign Material

        A wide variety of road safety awareness campaign materials can be produced with a variety of road safety messages. The materials include posters, brochures, stickers, booklets (also containing stories), and banners. Samples can be found by clicking here: All aim at specific target groups and their desired safe road user behaviour.

        • Uvukaji Barabara

        • Kivuko Chaalama ya Pundamilia

        • Waendeshe Baiskeli

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