Speed change behavior on combined horizontal and vertical curves: driving simulator-based analysis

Road alignments that combine horizontal and vertical curves on freeways have effects on drivers' speed changes that can lead to safety problems. This study examines speed changes on four types of combined curves: downslope, upslope, crest and sag. From design blueprints for a mountainous freeway under construction in Hunan Province, a total of 70 combined curves were programmed into the Tongji University driving simulator. Study participants drove through the simulated freeway while vehicle operation data was continuously captured. Speed changes on the combined curves were determined by calculating the differences between minimum and maximum speed values, and were classified into three behaviors: substantial speed decrease, steady speed (minimal change), and substantial speed increase. Multinomial logit models were used and the marginal effects of each variable were calculated in order to examine the effects on speed change behavior of each combined curve type and their adjacent segments. Results show that: 1) the speed change behaviors on the four types of combined curves differ in frequency, 2) the significant effects of geometric design characteristics on speed change differed by type of combined curve, and 3) design characteristics of adjacent segments also have significant and varying effects on speed change. Combined curves should therefore be studied separately, and their adjacent segments should be considered when combined curves are planned.