Traffic volume is one of the fundamental types of data that have been used for the traffic control and planning process. Forecasting needs and efforts for various applications will be increased with the deployment of advanced traffic management systems. With the importance of the short-term traffic forecasting task, numerous techniques have been utilized to improve its accuracy. The use of the subset autoregressive integrated moving average (ARIMA) model for short-term traffic volume forecasting is investigated. A typical time-series modeling procedure was employed for this study. Model identification was carried out with Akaike’s information criterion. The conditional maximum likelihood method was used for the parameter estimation process. Two white noise tests were applied for model verification. From the analysis results, four time-series models in different categories were identified and used for the one-step-ahead forecasting task. The performance of each model was evaluated using two statistical error estimates. Results showed that all time-series models performed well with reasonable accuracy. However, it was observed that the subset ARIMA model gave more stable and accurate results than other time-series models, especially a full ARIMA model. It is believed that the use of a subset ARIMA model could increase the accuracy of the short-term forecasting task within time-series models.

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Prediction and estimation of speeds on two-lane rural highways are of enormous significance to planners and designers. Estimation of speeds on curves may be easier than prediction of speeds on tangent sections because of the strong correlation of speeds with a few defined and limiting variables, such as curvature, superelevation, and the side-friction coefficients between road surface and tires. On tangent sections, however, the speed of vehicles is dependent on a wide array of roadway characteristics, such as the length of the tangent section, the radius of the curve before and after the section, cross-section elements, vertical alignment, general terrain, and available sight distance. Few studies have dealt with this issue because a considerable database is necessary to identify any significant trends and substantial modeling effort is required. Research analyzed the variability of the operating speeds on 162 tangent sections of two-lane rural highways, and models were developed for prediction of operating speed based on the geometric characteristics available. A one-model approach was used initially. Because of the low R2 values, a family of models was developed that better predicted operating speeds.

At some locations, the posted speed limit, based on 85th percentile operating speed, exceeds the roadway's design speed. This situation is a result of the fact that criteria used in highway design incorporate a significant factor of safety, that is, roadways are designed for near worst-case conditions. When this situation occurs, however, liability concerns can arise although drivers can safely exceed the design speed. A mail-out survey for state, city, and county transportation departments was conducted to document concerns and difficulties the engineering profession is experiencing with the relationships among design speed, operating speed, and posted speed. The factors most frequently considered when selecting design speed are anticipated operating speed and/or posted speed; however, these were not the most common factors selected by respondents. Urban versus rural and functional class were selected more often by the state respondents, whereas cross section was selected more often by the city/county respondents. Most respondents indicated that a ball bank indicator is used to set advisory speeds on horizontal curves. Many respondents expressed the desire to have a new procedure for selecting advisory speeds. Several agencies acknowledged that they have or have had sites with an operating speed greater than the design speed. The most frequent action taken was to install advance warning signs. Few respondents stated that they reduce the posted speed limit to match the design speed. Although few lawsuits involving the posted speed/design speed issue have occurred, liability concerns still exist. Several respondents provided comments that indicated their concern with current definitions and procedures.