In The Godfather, knowing that Sonny was a hothead and would immediately go to Connie's house after Carlo beats her up, he is ambushed at the highway toll booth by (presumably) Tattaglia men.
The ambush they have in place seems unnecessarily elaborate: involving at least eight men, seven of whom are definitely armed with automatic weapons (Tommy guns); two booths acquisitioned by the assassins and at least one car. Although the failure at killing Vito earlier (by just two guys armed with pistols) would have made the plotters more careful concerning the possibility of failure - surely this overkill in relation to Sonny was totally unnecessary and if anything, a liability?
This had been slightly puzzling me for a long while but (having not read the book) I believe I've worked it out. As much as a hothead Sonny was, the plotters didn't expect him to fly off the handle to such an extent as to go without protection. While one feels that Sonny made a terrible mistake leaving without the armed protection/escort (that Tom unsuccessfully sent out to follow Sonny); ultimately it would have made no difference. The ambush was extensive enough to cover any normal protection the Corleones would presumably have had - and had the car of Corleone men been right behind Sonny at the time of the ambush (shown in the photo below), would have almost certainly met the same fate.
They didn't know how many people (Sonny plus henchmen) they would be dealing with.
Sonny's reputation went before him; they could have reasonably expected him to come out shooting.
They wanted to not only kill him, but send a message to his family by disfiguring his body beyond recognition so as to deny him an open-casket funeral. (Vito, however, was owed a large favour by his local undertaker).
I always thought this was more of an homage to the St. Valentine's Day Massacre; the quintessential Mob statement. Yes, they might have assumed Sonny would come with more backup (that backup did arrive, minutes too late), but it was more likely they chose to use a show of force in a remote location, much like Capone's classic hit. Similarly, the hitmen were able to escape without being seen by a multitude of witnesses.