I had this problem (I still have it) and I did some pretty extensive testing (in my own opinion) and found some interesting results. You can read my posts over at MTS2 in the help section, or you can just read the result here
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I have found what is the cause of my problem! (sort of)
It is indeed something in my meshes folder, but I haven't found out yet. I have found out One very important thing though. When I buy a car for my sim, assign it to him/her (tried this 10 times), the first drive to a community lot always results in a taxi ride home. However, the second time in the same car, she/he can return home in his/her own car! But if I sell the old car, buy a new one and assign it to that sim, the first ride downtown results in a taxi ride again, and the second time I go, I can use my car again!
I uninstalled the whole game (all ep's) and reinstalled it, patched it, but only put in my hacks, reinstalled CEP, mesh folder, recolors folder (not the skins just yet), but I didn't try my neighborhoods. I created a custom neighborhood from the list. Then I created new sims everytime I put in a folder and tried taking them downtown.
testing (1)
Clean game- sim(a) can go downtown, can return home no problems.
Just hacks- sim(a) and sim(b)(newly created) can go downtown, can return home no problems.
hacks+CEP+meshes- sim(a) and sim(b)(newly created) can go downtown, can return home no problems. BUT! sim (c) newly created cannot return home in car, so I make him take taxi home. Then I again direct him to go downtown in the same car, and he can go home from there this time.
testing (2)
hacks+CEP+meshes- change car (sell and buy higher models)for sims (a,b and c), assign them to sims, go downtown- return home in taxi. Then try again after returning home, successful!
I think before when I first came across this problem, I always quitted to neighborhood without saving and tried different families all the time. Now I'm gonna see if my old neighborhoods can be saved, maybe at the weekend I'll try and see what causes this problem or maybe next week
**** I'm back. I put my custom neighborhood in from my backup file (it was an after- nightlife neighborhood so it's totally compatible, just the car problem), and the same result, the second time my sim goes downtown she can go home in her car, With a date even! I'm soo happy it finally works, even if it doesn't work the way it's meant to. I can see it maybe as a 'drunk-take taxi home' or 'new car too scared to drive it back home' incident
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If you have any questions, pm me, as I'm not around much these days