Not too difficult to do the conversions. I live near the Canadian border and drive in Canada frequently. 100Km/h = 60mph so I just multiply whatever I see on the road sign times .6 as my eyes are too old to read the fine print on the speedometer, which actually has kmh on the dial but in much smaller font.
A meter is .9 yards - 3ft in a yard so a yard is roughly close to a meter, Therefore a 9ft. snake is about 3 meters.
100mph - add about 60% = 160kmh
I drive in Canada a lot too and the Canadians are kind enough to make it easy for us. Since I drive in Canada AND suck at math, I've developed a few tricks for how to ballpark the conversion.
In city streets it's normal to have a maxiumum speed limit of 50km/h, and on highways, it's frequently 100km/h. These are the two easiest conversions to multiply by .6, yielding 30mph and 60mph respectively, so having them also be the two most common road postings is convenient and helpful.
Now when it comes to "ballparking it," it's easy to use those two landmarks (50/30 and 60/100) to approach other values. Adding or subtracting 5 mph for every 10 in difference from one or the other "milestones" will usually have you in the ballpark and with speed, the ballpark is fine. So for example, if the sign says "maximum 70," it's accurately 42, but you won't be pulled over for going 40 (30 +5 +5). Highway speeds of 110km/h mean you should be travelling at around 65mph. Since absolute accuracy is not required, ballparking it is both easy and close enough, especially if you round down.