Acceleration Characteristics of Starting Vehicles

Acceleration characteristics of starting vehicles are needed for many transportation analysis and design purposes involving driveways, turning bays, intersecting streets, traffic signals, railroad crossings, simulation software, and so forth. Constant acceleration is sometimes assumed, or AASHTO Green Book values based on piecewise-constant accelerations are sometimes adopted. However, continuing research has shown that linearly decreasing acceleration rates better represent both maximum vehicle acceleration capabilities and actual motorist behavior. It is not acceleration that usually is constant during speed changes; it is the rate of change in acceleration. The evolution of AASHTO acceleration values for design is traced, and the literature is reviewed for pertinent field observations. Linearly decreasing acceleration (and some deceleration) model parameters are calibrated for various sets of observational and design data. Despite published results from thousands of field measurements, the values selected for design were found to be based mostly on vehicle performance capabilities, with conjecture about how many of these capabilities are normally used by drivers. Design accelerations were found to deviate substantially from observed accelerations. At the start of motion, observed accelerations were about 15 percent faster for passenger cars and 45 percent faster for SU (single unit) trucks than design accelerations. As speed increased, observed accelerations dropped three to four times faster than design accelerations for these vehicles. Observed accelerations for WB-15 trucks began 40 percent to 75 percent slower than design accelerations, and they were considerably slower than the values in the Green Book for trucks at railroad crossings. Voids in the available information base are identified. Linearly decreasing design acceleration rates for motorists operating different classes of vehicles in different design situations are recommended, and revisions for Green Book parameters and charts are suggested. The recommended acceleration model greatly simplifies acceleration parameters but implies using different acceleration relationships for different design speeds instead of one for all.