(Center Identification Number: 79060-02-A)
Program Director, Center for Surface Mobility Applications & Real-Time Simulation environments (SMARTSe)
Upper Great Plains Transportation Institute
North Dakota State University
Practitioners use the term ride quality to indicate the degree to which a vehicle protects its occupants from factors that decrease ride comfort. Hence, factors that affect ride quality are numerous. The road impact factors (RIP) are uneven road surfaces and roadway anomalies such as potholes, cracks, and utility covers. The driver impact factors (DIP) are driving behaviors such as abrupt braking, rapid acceleration, weaving, and speeding around curves.
RIF and DIF can produce motions and noises that cause rider discomfort. The vehicle impact factors (VIF) affect how riders perceive those disturbances. VIF depends on vehicle suspension and handling characteristics, furniture design, interior aesthetics, and other invehicle features. Together, these factors result in the ride quality experienced.
Highway agencies narrow the definition of ride quality to the RIF and use it as a key indicator of pavement performance (Bridgelall 2014). Manufacturers design suspension systems to attenuate vibrations at frequencies that could cause human discomfort or affect handling safety. Humans are most sensitive to vibrations between 4 and 8 Hertz (Griffin 1990). Hence, nearly all suspension systems attenuate vibrations in that frequency range (Jazar 2008). However, variations in road roughness, driver behaviors, vehicle handling, and suspension design could result in peak vibration levels that induce significant levels of discomfort for some riders.
Studies have established that there is a strong linkage between ridership and the perception of service quality in terms of comfort (Benjamin and Price 2006). An old study found that the subjective rating of ride comfort was highly correlated to the frequency and level of vibrations experienced (Park 1976). Since then, relatively few studies characterize ride quality or its importance for transit bus riders. One study (Peterson and Molloy 2007) found that poor ride quality was a major issue of customer concern. Poor ride quality could be a significant deterrent for transit riders but the level of significance is unknown. Furthermore, the degree to which perceived ride quality correlates with the individual factors that causes roughness is unknown.
This research will develop a low-cost approach to measure the transit ride quality (TRQ). The methods will include developing the theoretical foundation to characterize the performance of the TRQ measure. The case studies will produce TRQ-indices for at least two transit routes of distinctly different levels of roughness. A survey of the riders on those routes will reveal the relationship between subjective perceptions of roughness and the objective levels measured. The survey will also reveal any influences from the actual roughness level experienced to the stated level of its importance. Agencies will benefit from the research products by utilizing the lowcost tool to study ride quality throughout their service area. Practitioners will have the ability to relate objective measures of ride quality to factors that could potentially affect ridership. The new method of ride quality characterization will provide an estimate of the relative contributions from road impact and driver impact factors.
The goal of this research is to discover any relationship between the stated levels of importance of ride quality and the actual or perceived levels of ride quality experienced. The objectives to achieve this goal are as follows:
1) Develop an new method of objective TRQ characterization
2) Measure the TRQ for at least two bus transit routes of distinct roughness differences
3) Assess the perceived level of ride quality for the selected routes
4) Assess the importance level of a smooth bus ride
5) Discover any relationship between the perceived level and objective measures of ride quality
6) Determine any relationship between the perceived and measured levels of ride quality and the stated level of its importance
The International Roughness Index (IRI) is the most widely accepted method to characterize ride quality objectively from road impact factors. It is the accumulation of the absolute rate differences between the sprung- and unsprung-mass of a reference quarter-car moving at a fixed speed of 80 km/h (Gillespie 1981). Producing the IRI requires special instrumentation to measure the elevation profile of a wheel path, and simulation software to transform that data into the index (Janoff 1990). Hence, the IRI has several important limitations for this study. The first limitation is that the simulation of a fixed quarter-car does not reflect the vibration modes induced in the actual vehicle. The second limitation is that the fixed simulation speed of 80 km/h does not reflect the effects of road roughness at other speeds. The third limitation is that the IRI does not reflect roughness from driver impact factors. Furthermore, most transit agencies do not have the budget and expertise required to obtain and operate the special instrumentation needed to measure the elevation profile of the selected bus routes.
Ride quality data is not available from any transit agencies. Hence, this research will develop a new and significantly lower-cost method of producing the TRQ by processing temporal, inertial and geospatial position (TIG) data logged with a smartphone. The PI has already demonstrated viability of the method to produce the RIF and the VIF from TIG data (Bridgelall 2014). However, producing the DIF and the TRQ requires further development.
Achieving the research objectives will require completion of the following work plan:
- Panel: develop an advisory panel consisting of potential transit stakeholders
- Literature: conduct a literature search of TRQ characterizations
- Software: develop the new method and models to produce the TRQ
- Data: collect TIG data from the selected bus routes
- Survey: design and implement the ride quality perception survey
- Models: use the new models to produce the RIF, DIF, and TRQ from the collected data
- Analysis: plot and evaluate the data to discover any relationships
- Report: prepare a final report of the results of the implementation and analysis
The PI will conduct the bulk of the research tasks. He will complete the literature review, develop the theories and new methods, develop the software, manage the data collection process, analyze the data, produce visual representations of the TRQ data, assess the research findings, and write the final project report. He will mentor and guide the RAs through the smartphone installation, operation, and data upload tasks. He will also coordinate with the project advisors and interact with transit agency officials to obtain the relevant permissions for TIG data collection and opinion surveys. The PI will work closely with the project advisors to obtain inputs for the survey design. The RAs will obtain guidance from the PI for distributing and collecting the questionnaires while riding the transit bus.
The project will begin in July 2015 and conclude in June 2016.