A tough question.And a longwinded answer.
I can Only give suggestions for North and Central America.
You will need this insurance in Canada,USA where it can be quite expensive, several hundred dollars if you spend a long time here.
Mexico cover can be obtained for about $85 for a whole year of "drivers license policy" which is cheaper than buying a personal policy for even 3 weeks.
In Central America it is compulsory in Nicaragua,Costa Rica and Panama and these countries ( also Belize , but not on the Panam) sell it at the border at affordable rates,you won't get in without it.And they do NOT recognize any policy you may have which says it covers these countries, although such a policy will give cover in addition to their national policies.
For Honduras,El Salvador and Guatemala such insurance though recommended is not compulsory and it is never asked for. Obtaining it is a problem, the local insurance companies will not deal with tourist policies so you will have to find a source ( expensive $148US for a month as one example) before you get here,as in Sanborns Insurance Brownsville TX branch only.Or you could do without and ride VERY carefully and not run over anybody. If you do prang anywhere they will find some way to assign blame to the responsible party or the "sucker" party i.e. the tourist who is not fluent in Spanish ,unfamiliar with the system and it s rules and afraid.
As a case study I'll give a tale of a personal experience from a car trip to Panama earlier this winter.
In Chiquimula Guatemala, I was involved in a minor collission with a pickup truck when that driver passed me on the right on a speed hump on a type of roundabout around a small city park/plaza and then cut left across my front.I hit the brakes but a light contact was made between my R headlight and the L rear flank panel of his truck box. Light did not break.
He immediately stopped and pointed out the dent I alledgedly had made there and he insisted that I make financial restitution so that it could be repaired.His truck was not new and sported many other dents and scratches unrepaired.He wanted money or we would have to involve the police,he said, and that would be a big problem .He spoke onlySpanish and said he had a friend in the bodyshop business who he could phone right away so that he might come and give an estimate of the repair job cost
I looked at the dent and saw BLUE paint in its center. MY car is BLACK. Callthe Police ,I said. Within minutes 3 police vehicles arrived and we had a lively discussion in which I explained everything and pointed out the BLUE paint. Yes, the cops said , the other driver did have the right of way as he came from a priority street. BUT, said I , Iwas already in the turn when he overtook me and then barged in front of me, there is a certain amount of reason and discretion to be applied when claiming "right of way".
The cops never once asked if I had liability insurance, which I did.
After I pointed out theBLUE paint and all the other old dents, the cops took the other fellow aside and when he came back he could speak English and he said he wasn't after my money, just the principle that I should be more careful (!) and not try to bully other traffic just because I was a gringo. We shook hands and he left .Had I not been adamant and stood my ground it could have been expensive
Thankyou Chiquimula Police Dept, handshakes all around.
This was only a minor incident,Had it been a major crash and if I had been at fault my insurance would have saved me a lot of grief.
If one does not have insurance it could wind up in jail time until some settlement is arranged.