Browsing by Author "Nagui M. Rouphail, Committee Member"

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Collision Models for Multilane Highway Segments Incorporating the Effects of Curbs
(2008-05-16) Baek, Jongdae; Jason A. Osborne, Committee Member; Billy M. Williams, Committee Member; Nagui M. Rouphail, Committee Member; Joseph E. Hummer, Committee Chair
The main objective of this study was to develop valid statistical collision models for multilane highway segments with or without curbs. For this, road geometric data, traffic data, and collision data for the three years were collected. The data include 2,274 collisions and 885 injury collisions that occurred on 191.85 miles of 199 directional segments. A new modeling method of introducing variables into the model one by one in a multiplicative form was applied. A nonlinear optimizing algorithm for estimating parameters using a negative binomial log likelihood function was adopted for the modeling. The functional form of the variable to be introduced was determined based on the relationship between the recorded number of collisions and the number of collisions predicted by the current model without the variable. The integrate-differentiate method was applied to find candidate functional forms for each variable. Model selections were based on the -2 log likelihood and BIC statistics. The cumulative residuals (CURE) plot method was adopted for checking the goodness of fit of the models. As a result of the modeling efforts, the annual average daily traffic, access point density, shoulder width, and shoulder type variables were introduced to the final model for total collisions. The same variables except the shoulder type variable were introduced to the injury collision model. Overall, then, it appears that curbs mean fewer total collisions and no change in injury collisions as compared to no curbs on the sampled road segments. The models developed in this study were based only on the data for North Carolina and limited number of variables. The developed models can be improved in the future by collecting data on more miles, by bringing more explanatory variables into models, and by using the data from other states. Additionally, the characteristics of vehicles speeds on multilane highways were analyzed and compared. The results showed that the mean speeds for the non-curbed sites were about 2 to 3 mph higher than those for the curbed sites.
Comparing Operation and Safety between a New Nano Interchange and Conventional System Interchange
(2008-12-19) Moon, Jae Pil; Billy M. Williams, Committee Member; John R. Stone, Committee Member; Joseph E. Hummer, Committee Chair; Nagui M. Rouphail, Committee Member
The primary purpose of this research is to estimate the capability and applicability of new nano interchange designs as they compare to conventional four-level interchanges. Nano interchanges were conceived to provide drivers with high speed and short travel distances while requiring less right of way in dense urban areas where real estate is expensive. This research consists of two tasks: (1) operational evaluation and (2) safety evaluation. The operational evaluation compares measures of effectiveness (MOEs) between nano interchanges and conventional interchanges for thirty volume scenarios comprised of varied data for through volumes, ramp volumes, and percentages of heavy vehicles. The estimations were conducted for an entire interchange and for key freeway segments. The MOEs for an entire interchange are travel time, speed, delay time, and ramp travel time; and the MOEs for key freeway segments are density, speed, and level of service. The safety estimations were made by building safety prediction models for collisions that may occur in the influence areas of ramps. These safety models are based on negative binomial distribution and quantified safety effects of traffic, geometric, and environmental factors using three statistical methods: (1) a generalized linear model with only main effect variables, (2) a generalized linear model with main effects and interaction terms, and (3) the new Hauer method model. Specifically, this study addresses the safety effectiveness of left-hand ramps, which are needed in the nano interchange designs, because the safety effects have not been well quantified in previous studies. Considering goodness-of-fit statistics such as the likelihood-ratio test, Akaike?s information criterion (AIC), ordinary R-squared values,the Miaou method, and CURE plots, the best safety models were chosen for four collision categories: (1) Total collisions of all types, (2) Severe collisions, (3) Related Total collisions,and (4) Related Severe collisions. The operational evaluation for an entire interchange shows that conventional interchanges perform better than nano interchanges for the volume scenarios tested. The analyses of key freeway segments show that most of operational difficulties for the nano interchanges are in diverging influence areas located on upgrade segments. The analyses also indicate that several merging influence areas that are connected to ramps with steeper and longer upgrades also have lower performance levels. The safety prediction model developed unique linear or non-linear relationships among traffic, geometric, and environmental factors. Left-hand ramps appear to have higher collision frequencies than right-hand ramps, and on-ramps have higher collision frequencies than off-ramps. In addition to estimating the safety effects, this study compares three modeling procedures. This research shows that the Hauer procedure sufficiently represents linear and non-linear relationships in terms of diverse functional forms by each explanatory variable, whereas a generalized log-linear model does not adequately develop linear relationships for some explanatory variables in terms of linear functional forms. However, the generalized log-linear model with interaction terms among independent variables fits to data as well as the Hauer procedure. Finally, based on these estimations, the operational and safety problems of nano interchanges are addressed in this study, and the following recommendations are made to improve the operation and safety effectiveness of the interchange: (1) deceleration or acceleration lanes could be extended; (2) a two-lane off-ramp or on-ramp for the nano interchange could be considered; and (3) left-hand ramps could be replaced by right-hand ramps.
Empirical Collision Model for Four-Lane Median Divided and Five-Lane with TWLTL Segments
(2004-04-08) Phillips, Stacie Leigh; Joseph E. Hummer, Committee Chair; Roger Woodard, Committee Member; Nagui M. Rouphail, Committee Member
There is a great debate between the use of raised median and two-way left turn lane (TWLTL) treatments on arterial roads. The raised median is generally believed to be the safer of the two treatments for both vehicular and pedestrian traffic, while the TWLTL is considered more beneficial to businesses since it allows direct access to left-turning traffic from an arterial. The North Carolina Department of Transportation (NCDOT) has been using studies conducted in other states as well as collision rates from case studies in North Carolina to make decisions in such matters. Studies conducted in other states, however, are generally not considered relevant to North Carolina drivers and the use of collision rates is criticized because they do not account for driveway densities or land use. This research was conducted to definitively determine which of these cross-sections is safer on North Carolina's highways and under what conditions. A random sample of 200 four-lane roads with raised median and five-lane roads with a TWLTL was visited and data were collected on the geometric, volume, land use, and collision characteristics of each site. Of these 200 sites, 143 were found to meet all necessary criteria and were used to calibrate and validate a collision model for each cross-section using Poisson regression procedures. The form of the collision models developed by Bonneson and McCoy were used since they were judged to give logical and consistent results. The final regression equations showed that the factors contributing to collisions on these two segment types were average daily traffic (ADT), length, predominant land use, and approach density. For predominantly residential and industrial land uses, the raised median design was found to be associated with fewer collisions than the TWLTL across all values of traffic volume and approach densities. The raised median design also has a safety advantage over the TWLTL for predominantly business or office land uses with low to medium approach densities (0-25 approaches per mile). For business and office land uses with medium to high approach densities (25-90 approaches per mile), the TWLTL appears to be safer at low traffic volumes and the raised median appears to be safer at high traffic volumes.
Measurement and modeling of the activity, energy, and emissions of conventional and alternative vehicles
(2009-12-07) Choi, Hyung-Wook; Donald R. van der Vaart, Committee Member; Nagui M. Rouphail, Committee Member; H. Christopher Frey, Committee Chair; E. Downey Brill, Committee Member
Since the transportation sector is a significant contributor of air pollution, the capabilities of estimating fuel use and emissions for various vehicles is important to air quality studies as well as the development of environmental guidelines and policy recommendations. In this thesis, a common or similar modeling approach based on second-by-second data using portable emission measurement system (PEMS) was developed to estimate energy and emission estimation for a wide variety of on-road and non-road sources with conventional and alternative technology. Based on vehicle-specific power (VSP) and speed-acceleration modal models, two correction factors were developed to estimate fuel consumption and emissions for vehicles which were driven with high and constant speed on highway. The corrected emission factors for NOx, HC, CO, and CO2 were significantly higher for high speeds and lower for low speeds than base emission factors estimated using MOBILE6 which is based on transient test cycles with durations on the order of 10 minutes. A similar methodology was used to estimate energy use and emissions for a plug-in hybrid diesel-electric school bus (PHSB) and conventional diesel school bus (CDSB) for typical school bus routes in NC. To quantify the reduction of fuel use and emissions between PHSB and CDSB for same driving routes, the mixed-modal models based on manifold absolute pressure and VSP versus emissions were developed. Plug-in hybrid technology showed significant emission reductions for stop-and-go driving pattern. These results could provide a support for transportation and air quality management. This thesis also introduces a simplified emission estimation methodology for locomotives based on rail-yard measurements using PEMS. This alternative measurement method is faster and cheaper than a federal reference method (FRM). The fuel-based emission rates based on PEMS measurement were comparable to FRM. It should serve as a useful basis of comparison to data in future measurement campaigns.
Simulation of Pedestrian-vehicle Interactions at Roundabouts.
(2008-01-05) Chae, Ko Sok; Nagui M. Rouphail, Committee Member