TTC calculation

[whatgit]

Hello,

I am wondering how do you calculate the TTC in the dataset? I was assuming that
TTC = dhw/(xVelocity - precedingXVelocity); where TTC is time-to-collision, dhw is the distance headway, xVelocity is the ego vehicle's velocity, and precedingXVelocity is the preceding vehicle's velocity.

However, when I check it with the data, it was not correct. Therefore, I am wondering how do you calculate TTC in the dataset?

Please see the examples below.

Example from dataset_01

Case 1: vehicle id 2 is following vehicle id 13, here are the parameters in "track_data" of the vehicle id 2:

dhw = 112.62
ttc = -17.38
thw = 3.51
precedingXVelocity= -38.53
xVelocity = -32.06

If we follow the equation above. TTC should be equal to 112.62/(32.06-38.53) = 112.62/-6.47 = -17.40
(I think minus sign here indicate driving direction so I will ignore it for now)

Case 2: vehicle id 8 is following vehicle id 2, here are the parameters in "track_data" of the vehicle id 8:

thw = 2.29
dhw = 72.05
ttc = -104.67
precedingXVelocity = -32.04
xVelocity = -31.36

If we follow the equation above. TTC should be equal to 72.05/(31.36-32.04) = 72.05/-0.68 = -105.95