Determination of the effect of speed hump and bump profiles on urban roads in terms of ride comfort via field tests, numerical model and analytical model

Speed humps and bumps that have a widespread of usage all over the world are among the most efficient traffic calming devices to prevent urban traffic accidents by forcing drivers to lower their speeds to moderate levels. If drivers do not reduce their speed in accordance with the speed limits determined by local authorities when they encounter speed humps; the driving safety and level of comfort perceived by the driver and passengers shall definitely reduce. As a result; speed humps and bumps have a deterrent role on driver's desire of speeding up. On the urban roads of Turkey, every year great number of traffic accidents resulting with fatalities and serious injuries are being recorded. Pecuniary damages that happens due to those traffic accidents is the another aspect of the problem. The reason of the majority of traffic accidents that happened on urban roads is over speeding. Physical measures to be taken in order to take the vehicle speeds under control especially on the road sections where pedestrian traffic is dense, will definitely be fruitful under the current conditions of Turkey. Traffic calming methods that have widely being used all over the world is the most effective way to ensure it. Among the techniques it includes, there is also physical measures that regulates the traffic flow. The most important, effective and commonly used physical precautions are speed humps and speed bumps with optimum profiles to be constructed with reasonable distances on proper sections of urban roads. Getting a remarkable reduction in the number of traffic accidents that occurred on urban roads with material damage, fatality and injury will contribute to the economy of the country in a dramatically manner as well as preventing a great number of pedestrian, passenger, motorist and cyclist death and permanent disabilities. A well designed speed hump should reduce the vehicle speeds to the speed limit range, but at the same time it should not jeopardize the ride safety and ride comfort. Vertical acceleration that arise during the crossing period over a speed hump is the most important factor that effects ride comfort and safety. The main purpose of this study is to investigate the effect of speed hump and bump profiles on ride comfort and design optimization under variable vehicle speeds as well as making an attempt to establish an approach for the selection of optimum speed humps and bumps corresponding to all speed ranges taken into account in the course of experiments. The study has three main stages as field tests, numerical model (simulation) and analytical (mathematical model). The study is based on the evaluation of root mean square (rms) values in terms of ride comfort and driving safety that were calculated by means of previously recorded instantaneous vertical acceleration values. Following the introduction, a comprehensive literature review related to the subject takes part in. In section 2; thereafter the basic information given regarding traffic calming and traffic security; traffic calming devices including speed humps and speed bumps have been presented. In section 3; after referring to discomfort criteria and its evaluation, methods of vibration evaluation that commonly used in determination of level of ride comfort have been introduced. In section 4; during preparation stage of the thesis, by setting out the motion of a car wheel accelerated down an inclined plane onto a horizontal plane where it was allowed to collide with a speed bump; basic elements of design optimization and examination of performance of a speed bump have been demonstrated by means of an iconic model. This section in which includes a simple and practical iconic model can be esteemed as an introductory part to the essential scope of the thesis. Section 5 includes field tests and its evaluations. Field tests were performed on seven parabolic speed humps and a trapezoidal speed table within the boundary of Istanbul Technical University Ayazağa Campus Area. As a result of examination of the values which were obtained through on-site surveys by a surveying team, all speed humps are considered as parabolic in shape. However; minor surface irregularities and design defects were observed due to inattentive workmanship and lack of design. Tests were carried out twice with different loads as driver + 1 passenger and driver + 4 passengers by means of a 2014 model Volkswagen Golf passenger car. Five vehicle speeds have been taken into account within the range of 15 km/h to 35 km/h with 5 km increments. As the field tests were focused on the measurement of the vertical acceleration occurred when crossing the speed humps; during experiments instantaneous vertical accelerations have been measured and recorded with an accelerometer, a data logger, a Testlab Network Software and a PC. Thereafter; based on the previously recorded instantaneous vertical accelerations, rms values have been calculated. Basic evaluation method was selected in order to evaluate the vibrations occurred during the test period as none of the crest values exceeds "9". In section 6; besides basic information that was given regarding modelling; modelling techniques have briefly been presented. Section 7 consists of a simulation stage. A general purposed finite element simulation software Ls-Dyna 3.1 version were used for this phase. Though parabolic, circular and trapezoid profile types were considered for speed humps; only circular and parabolic types were taken into account for speed bumps in the course of simulation period. In this section, for variable vehicle speeds three sets of speed humps and one set of speed bump that consists of combination of different height and length values were considered. In section 7, a comparison of rms values which were calculated by means of instantaneous vertical acceleration values that obtained via field tests and rms values which procured during simulation phase from speed hump profiles identical with those in the field, were also accomplished. Thereafter, a correction factor has been assigned for each vehicle speed range that were taken into account during the test process. Despite the required efforts have been made specially to get the distinctive weight and suspension properties of the cars that was utilized for the field tests and the one represented in the simulation stage as closer as to each other by means of Ls-Dyna software settings, it has still become essential to assign such a correction factor as it would be incorrect to assume the behaviour of them identical. Some minor factors such as center of the gravity, tires, geometrical dimensions of the car, driver's behaviour, etc. are ignored within the scope of this study. In this section finally; optimum speed hump and speed bump profiles have been determined among the ones that were already tested during the simulation stage for each considered speed range. In the approach which has been developed for the assignment of optimum speed hump profiles, upper limits of rms range given in ISO 2631-1 were considered and it has been assumed that it would be sensible to take the rms value of 0,63 m/s2 into account that corresponds to the threshold point of "a little uncomfortable" and "fairly uncomfortable" level. The main reason in accepting the value of 0,63 m/s2 has been based on the the strategy that when the speed limit exceeded during the period of crossing over a speed hump, the sense of discomfort perceived by the driver would increase, on the contrary; it would become to moderate levels below the speed limits and as for crossing the speed hump at the threshold of speed limit, the driver would feel some discomfort as an alert in order to deter him from the tendency of speeding up. On the other hand, as the length of speed bumps are dramatically shorter than speed humps, the duration of crossing over them is fairly short. For that reason, since the rms values shall not be representative and reliable; maximum vertical acceleration values have been used for the evaluation of speed bumps. In that case, it would be reasonable to take the rms value of 1,00 m/s2 into account that corresponds to the threshold point of "fairly uncomfortable" and "uncomfortable" level as the maximum vertical acceleration values are the peak values. In line with this approach; corrected rms values belong to already tested speed humps were examined and optimum profiles among them have been selected. Section 8 contains mathematical model process in which rms values were calculated by dint of instantaneous vertical acceleration values that were determined by a quadratic differential equation named as "Basic Equation". Solution of basic equation has been attained by 11.0 version of the software "Wolfram Mathematica". In this section, solely parabolic type speed hump and speed bump profiles for variable vehicle speeds have been taken into account. Damping coefficient, spring stiffness constant, vehicle mass, test duration, height and length of speed humps and speed bumps as a function of time have been placed into the basic equation, resolved by means of the software and finally rms values have been calculated with the aid of instantaneous vertical acceleration values that previously were obtained. Thereafter, it has been investigated whether the results obtained in this section supports those acquired during the simulation stage or not. The ninth and last section covers conclusion and recommendations. In this section, information has been given related with the results of the study and suggestions have been made for the similar researches to be probably fulfilled in the future. The effects of speed hump and speed bump profiles on ride comfort and driving safety for variable vehicle speeds have comprehensively been investigated in this study. The study is based on the evaluation and comparison of vertical acceleration data obtained through the field test and the simulation stage. Establishment of a relationship defined as "correction factor" between the speed hump profiles tested on the field and those tested during the simulation stage under certain vehicle speeds that experimented in the course of simulation phase with far more comprehensive variety, will make the application of speed hump profiles possible onto the field. This issue is one of the most important result of this study. Field tests, simulation and numerical model results have shown that vehicle speed is the most important parameter that effects ride comfort and design optimization as emphasised in the most of similar previous studies. A strong negative correlation between vehicle speed and ride comfort has been determined. It has been observed that when crossing over a speed hump, as the vehicle speed increases, the discomfort level that caused by higher vertical acceleration values also increase. It has also been concluded that speed hump and speed bump height is also an important parameter that influences on ride comfort and driving safety. However, as it would be inadequate to assess the ride comfort solely with the height of speed hump, it is recommendable to consider other parameters of hump profile such as profile type, hump length, etc. and vehicle properties, as well. Another finding of both field tests and simulation results is that as the weight of the vehicle increases, vertical acceleration that impacts to the vehicle and in line with this reality, the sense of discomfort perceived by the driver decreases. A negative relationship between the sense of discomfort perceived by the driver and the vehicle weight has been perceived. It has also been observed that any increase in the length of speed humps provided that minimum hump length should be bigger than wheelbase length of the vehicle, has a slight positive effect on ride comfort. A further minor finding of the study is that precise application and construction of a well-designed speed hump profile leads to obtain more reliable and moderate rms values in terms of driving safety and ride comfort. Since only the ride comfort level of the driver is considered in this thesis study, the sense of discomfort perceived by the passengers has been omitted. In order to comprehend the subject more clearly, it may be useful to carry out a more detailed research including the passengers as well. It is highly recommended for the similar studies to be realized in the future that; the approach created for the selection of optimum parabolic speed humps in this investigation can be tested for other hump profile types such as sinusoidal, cycloidal, harmonic, etc. with distinctive speed ranges and hump geometrical features. It would also be more useful if the future studies comprise a satisfactory amount of speed humps in terms of variety of hump profile and a wide range of vehicle speeds when considering the field tests. In this manner, the approach and solution techniques used for the assignment of most suitable speed hump and speed bump profiles in the scope of this study, have a big importance in terms of lighting up the similar future studies and researches to be possibly realized.