You're missing the point... it's not possible to tell in advance whether someone should be arrested or not, you have to follow a standard procedure for a lot of reasons, not the least of which is that doing so safeguards everyone's rights.
In the case of traffic violations, the decision to not arrest by default is made far in advance as a matter of policy. When someone at a traffic stop gets arrested, it's not for the traffic violation, it's for another crime they're wanted for, and the standard process for that overrides the traffic stop process.