A kei car will always be unsafe for highways.
Why would anyone expect something from a car it wasnt designed to do? However that doesnt mean it is forbidden to take it on a highway. You will need to practice extra caution on highways, which imo is:
1. Dont drive fast. 100 kmph is fast for an Alto. Keep it under 100kmph.
2. Stay clear of buses, check rear view mirror frequently.
3. Avoid overloading.
Who remembers United Bravo and Prince pearl? They were cheaper than Alto still failed to make a mark.
