The IEDs are meant to penetrate the windshield and the shrapnel and 'small balls' coming out the IED are supposed to killed everyone inside. So even if the vehicle isn't much damaged the folks inside are indeed dead or severely injured. Let's talk about the injuries alone. The PENTAGON said that from the Iraq war at least 400,000 combatants from the occupation coalition got CRITICAL BRAIN DAMAGES!!! That's even worse than death (because they live excruciatingly torturous lives right up to their 'natural death'). These IEDs killed ten's of thousands (mostly contractors) and the US and their allies are hiding the casualties.

If the insurgents plant, say, 10 IEDs...8 are successful.

Iraqi insurgents were masters at IEDs. Especially the Shia groups.

PS - One point to note: the Shia Mujahideen were never against the Iraqi Army and Police formed after the fall of Saddam - the federal forces and the Shia Mujahideen never fought against each other. Even before the fall, they only used to attack specific Sunni Baathist scum within the police and Army.