To clarity drivers’ mental load and the influencing factors when negotiating the helical ramps and helical bridges, field driving tests using real passenger cars were conducted on four interchanges in the mountain city. The driver's ECG signals under the natural driving habits were collected by onboard instruments. Based on this naturalistic driving data, the relationship between heart rate amplitude, the RMSSD index overall distribution and the ramp radius and slope were analyzed. The results indicated that the driving environment and horizontal curvature of ramp, as well as the on-ramps junction and diversion are the main factors of the mental load. The following, overtaking and meeting in the driving process all increase the driver's mental load. The driver's nervousness is higher when they are driving on uphill ramps than that on the downhill ramps. There is a strong negative correlation between the ramp radius and heart rate variability. The RMSSD will increase to some extent as the radius of the ramp decreases. Moreover, the unskilled driver is most sensitive to the change of the ramp radius. There are two different correlations between ramp gradient and the heart rate variability: for unskilled drivers, a linear increase in RMSSD as the slope increases, while for the general and skilled drivers, their RMSSD profiles have a trend of low in the middle and high on the both sides.

urban traffic;driving mental load;heart rate variability;mountainous city