Johan Granlund, Page number 1(8)

DESIGN OF A SHOCK-FREE SPEED HUMP

Johan Granlund, Fredrik Lindström

Swedish National Road Administration, Consulting Services Pavement Engineering Division

Röda Vägen 1 S-781 87 Borlänge, Sweden

E-mail: johan.granlund@vv.se

Abstract Road bumps reduce vehicle speeds on residential streets and other densely populated areas, thus improving safety and comfort for pedestrians and bicyclists. Unfortunately, the effects on health and safety for drivers and passengers passing these obstacles are rarely considered by road agencies, consultants and contractors. Many current bumps induces harmful whole-body vibration and shock when passing, even at legal speeds. Injuries can be immediate, e.g. fracture of vertebrae, or long term, e.g. low back pain. Bus drivers in many cities pass up to 40.000 bumps per year. The bumps will also cause additional longitudinal and vertical stress to the vehicle. A literature review shows that previous research on the subject of road bumps has been severely misinterpreted. As a result, bumps are in many countries today designed to cause a high maximum vertical acceleration (shock) level. To cause shocks on purpose, is in obvious conflict with ergonomic knowledge such as in directive 2002/44/EC. In a project described in this paper, a “shock-free” speed hump has been designed using vibration engineering. The new hump will cause uncomfortable vibration at high driving speeds, but only a minimum of shock will occur when passing. A full-scale test will be done during 2003.

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INTRODUCTION Since the introduction of “Vision Zero” for road safety in Sweden, the construction of road humps/bumps has boomed. Complaints about ride related back pain among urban bus drivers have also boomed. In a handful cases, car and bus passengers have been severely injured when passing the obstacles; fracture of vertebrae, ruptured liver et c.

Road hump regulations and design codes in many countries -- Sweden and Denmark for instance -- specify that these obstacles should cause a 0,7 g (7 m/s2) vertical acceleration level at the design speed. A quick literature review reveals that this target is the result of a serious misunderstanding.

The famous Watt hump (1973) was designed on panel rating basis. Today there are demands for new designs that for instance suit disabled pedestrians better, such as a flat plateau. The introduction of the 0,7 g target changed the design focus, and opened up for quite sharp obstacles that induce high shock levels.

The objective of the work described in this paper, is to develop a hump that fulfils both new demands from disabled pedestrians and Watts original criteria for speed control without causing harmful shocks. Manufacturer Gunnar Prefab AB in Rättvik, Sweden, is owner of this “shock-free” hump design, and pattern protection is pending.

LITERATURE REVIEW

The Watt Speed Hump

Watts (1973) stated sound criteria for speed humps; "The ideal speed control hump should probably exhibit the following characteristics:

��At and below the design speed all drivers should be able to cross the hump without damage to load or vehicle, or loss of control and they should suffer no discomfort.

��Above the design speed the driver should suffer a degree of discomfort depending on the amount by which he violates the design speed but there should still be no damage to load or vehicle or risk of loss of control."

When testing various humps, Watts used panel ride quality ratings of the designs. This way he experimentally developed a 3,7 m long and 10 cm high arc-shaped hump, that was a great improvement to the bumps in frequent use at that time. A Watt hump is compared with a typical bump in Figure 1.

Watts also measured the peak value of positive vertical acceleration in the vehicles. This may have been in order to learn about the risk for damage and loss of control. For the best hump, later known as the Watt hump, peak acceleration was measured to 0.93 g in a small passenger car at 32 km/h.

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Figure 1 A typical bump, compared with a Watt hump. From Stephens (1986)

Confusing the Definition of Peak Acceleration

Alppivuori & Laitakari (1981) tested the 3,7 m * 10 cm arc-shaped Watt speed hump with large Nordic type vehicles.

Panel ride quality ratings was the main evaluation method. Also vertical acceleration was again measured in the test vehicles. However “peak acceleration” was defined in an erroneous way, see Figure 2. Thus their data refer to peak-to-peak acceleration, which for typical ride vibration is roughly twice as high as the positive peak acceleration measured by Watts.

Figure 2 Confusing peak acceleration with peak-to-peak acceleration. From Alppivuori & Laitakari (1981)

Alppivuori & Laitakari came up with 0.5 to 0.9 g “peak acceleration” (rather

peak-to-peak) in a passenger car driving at 30 km/h over the Watt hump. This corresponds to about 0.25 to 0.45 g true peak acceleration, which is more than 50 % less than what Watts measured. However the panel ride quality ratings matched well between the two studies. This is very interesting, since it tells something about the (lack of) correlation between peak acceleration measurement and panel ride quality rating. The Serious 0.7 g Mistake Stephens (1986) is referring to Watts study as well as the Alppivuori & Laitakari study. He writes "Best-fit analyses of discomfort and the peak vertical acceleration of vehicles passing over humps yield good-fit linear relationships. Assuming that a

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midpoint discomfort value is a threshold for speed control at the road hump, peak vertical accelerations of about 0.69 g (6.8 m/s2) will induce the typical driver to reduce speed."

With this work, Stephens caused a change of focus from panel ratings to peak acceleration measurements. The unfortunate change of focus, opened up for poor bump designs with large Slope Variance. Comfort Rating of Transient Vibration The manner in which vibration affects human is dependent on the vibration frequency content. Thus the vibration data must be frequency-weighted. This has not been done by neither Watt (1973), nor Alppivuori & Laitakari (1981).

Spång (1997) presents a comprehensive study of comfort ratings of vibration containing single event shocks. This work shows how different vibration definitions correlate with human ratings. The results show a large lack of correlation between transient peak acceleration and comfort (38 % for unweighted peak value). The best fit with comfort rating was derived for the running rms of weighted acceleration; 92 %. Other studies have verified the Swedish study; peak acceleration, especially unweighted, is a poor indicator for the degree of discomfort.

In ISO 2631-1 (1997) the running rms of weighted acceleration is recommended when evaluating human exposure of very transient vibration (a high crest factor).

Current Knowledge About Shocks Long-term exposure to vibration containing multiple shocks, such as when travelling over rough surfaces, bring an increased risk to the lower lumbar spine.

Exposure to shocks can bring very large health effects, while the risk is very difficult to measure. Directive 2002/44/EC states that in risk assessment, particular attention shall be given to any exposure of intermittent vibration or repeated shocks.

From acceleration data describing the number and magnitudes of peak compression in the spine, ISO/DIS 2631-5 predicts the response of the bony vertebral endplate (hard tissue) for a seated person in an upright position, while also dealing with effects of multiple shocks and the posture on the intervertebral disc (soft tissue). ISO/DIS 2631-5 also give guidance on assessment of health effects to the spine.

The AASHO Road Test In the early 1960’s, the huge AASHO road test was done. The objective was to find suitable taxation principles for trucking, and to develop know-how in pavement engineering. An important measure was the road surface’s “Present Serviceability Index”, PSI, that predicts the “Present Serviceability Rating” expected from a road user panel. The PSI-value is usually affected to more than 95 % by short-wave Slope Variance of the surface roughness at the constructed test roads. This strongly indicate how the properties of the road profile corresponds to ride quality.

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DESIGN OF A SHOCK-FREE SPEED HUMP

Slope Variance, SV, is the second (space domain) derivative of the road profile elevation, and it excites the second (time domain) derivative of the ride displacement; namely the vertical acceleration. Thus a shock-free speed hump must have a minimum of short-wave SV to avoid wheel axle hop shocks, but enough long-wave SV to cause resonance in chassis bounce at high speeds.

A simple tool for hump design layout is MS Excel®, where one easily can compute slopes and SV for a given profile. SV is then minimized over a step corresponding to wheel axle hop (about 10 Hz) at the design speed. For 30 km/h, SV shall be minimized over 4 dm length. After a couple of hours of work in Excel®, a layout for a shock-free hump design is prepared, see Figure 3. To avoid resonance in vehicle pitch, the rear ramp has a very low gradient. This makes the hump asymmetric, thus it requires an channelising island on streets with opposite traffic. Another possibility is to extend the plateau to at least 8 m (longer than typical bus wheel base), and then use a mirror of the front ramp as rear ramp.

The software Ride Quality Meter®, RQM, is an advanced tool for evaluating road roughness, as well as speed hump profile designs. In Figure 4 a 10 cm * 5 m flat-top hump is tested with RQM, in this case with a quarter car simulation using “The Golden Car” chassis specification. The specification is developed by the World Bank as a standard for road roughness evaluations with quarter car simulations.

The result in the middle window of Figure 4 shows that peak weighted acceleration reach about 0.6 g (not 0.7 g!) at 30 km/h over the flat-top hump. In the lower window the running rms of weighted acceleration turn out to reach 2 m/s2, corresponding to “Extremely uncomfortable” according to the comfort scale in ISO 2631-1 (1997). Clearly the 10 cm flat-top hump is unnecessary tough for 30 km/h.

In Figure 5 results are given for a similar simulation over the shock-free profile showed in Figure 3. As seen in the middle window, peak weighted positive acceleration reach about 0.4 g. Thus it causes much less hazard to health, earning the name “shock-free”. In the lower window the running rms of weighted acceleration turn out to reach 1.4 m/s2, corresponding to “Very uncomfortable” according to the comfort scale of ISO 2631-1 (1997).

Real road vehicles have both front and rear axles. This brings a “smoother” chassis dynamics than shown in the quarter car simulation. Real vehicles can be well modelled with a “full car” simulation, where all wheels are considered. In Figure 6 the vertical acceleration (here no attention has been given to fore-aft or pitch vibration) results are given for a full car simulation of a 30 km/h drive over the shock-free hump. The chassis specification used is for a Chevrolet 3500 Mobile Intensive Care Unit Ambulance. The middle window shows vertical peak weighted acceleration of about 0.25 g. The lower window shows that the running rms of weighted acceleration reaches 0.8 m/s2, corresponding to “Uncomfortable” as per ISO 2631-1. This brings confidence in that the shock-free hump will be efficient in speed control.

In addition to vertical vibration and shock, speed humps also induce longitudinal and rotational shocks. The longitudinal loads affect vehicle wear, since vehicle suspension is designed mainly for vertical loads. Rotational shocks can

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occasionally add to vertical vibration and bring severe injuries to vehicle occupants. Tests not shown in this paper show a large decrease in such vibration when replacing traditional Watt humps and flat-top humps with shock-free humps.

The next development phase, running during the spring 2003, is to construct the shock-free hump in pre-cast concrete, and make full scale tests in reality. The tests will be done in various vehicles and speeds. After initial testing, large scale testing (incl. noise, ground born vibration, speeds et c.) will be done at a hand full of places in the Gothenburg area.

Slope Variance is often significantly affected by construction errors, wear and tear. If existing humps are accurately measured with a laser/inertial Profilograph, all SV can be evaluated with Ride Quality Meter®.

SUMMARY The widely used design target 0.7 g for speed control humps is not relevant at all. It is merely a product of scientific misunderstandings.

Ergonomic “shock-free” humps can have high profile elevation, high average ramp slope, high max. ramp slope, but must have low short-wave Slope Variance.

Various speed hump designs can be evaluated with the Ride Quality Meter® software, designed for road roughness evaluations. If profiles of existing humps are accurately measured with a laser/inertial Profilograph, also construction errors, wear and tear can be included in the evaluation.

Figure 3 Geometry of a shock-free hump

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Figure 4 Quarter car result for a 30 km/h ride over a flat-top hump

Figure 5 Quarter car result for a 30 km/h ride over a shock-free hump

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Figure 6 Full car result for a 30 km/h ride over a shock-free hump

REFERENCES

��Alppivuori, K. & Laitakari, P. The effect of a hump and an elevated pedestrian

crossing on vehicle comfort and control. Technical Research Centre of Finland, Road and Traffic Laboratory Report 69, Espoo 1981

��Cirkulære 188 om udformning af hastighedsdæmpende bump. Trafikministeriet, 2000 ��Directive 2002/44/EC on the minimum health and safety requirements regarding the

exposure of workers to the risks arising from physical agents (vibration) (sixteenth individual Directive within the meaning of Article 16(1) of Directive 89/391/EEC). The European Parliament and the Council, 2002

��ISO 2631-1. Mechanical vibration and shock. Evaluation of human exposure to whole-body vibration. Part 1: General Requirements. The International Organization for Standardization, 1997

��ISO/DIS 2631-5. Mechanical vibration and shock. Evaluation of human exposure to whole-body vibration. Part 5: Method for evaluation of vibration containing multiple shocks. The International Organization for Standardization, 2001

��Nilsson, U. Ride Quality in Ambulances – Modelling and Model Validation. TRITA-FKT 2003:XX

��Spång, K. Assessment of whole-body vibration containing single event shocks. Noise Control Eng. J., 45(1), 1997, pp 19-25

��Stephens, B.W. Road Humps for the Control of Vehicular Speeds and Traffic Flow. Public Roads, 1986

��The AASHO Road Test. Special Reports 61A-61E. HRB, National Research Council. Washington, D.C., 1961

��Vägutformning 94 (VU94). Del 5.5.6.4, Gupp. Vägverket, publ 113-128. 2002 ��Watts, G.R. Road humps for the control of vehicle speeds. TRRL lab. report 597,

1973