There is no statute that requires a company to waive your glass deductible in Massachusetts, so there is no "windshield law" as you referred to it.
If you have a glass deductible, you will have to pay it when you have a glass loss. It just so happens that in Massachusetts until recently the average personal car insurance policy did not come with a glass deductible in regards to your Comprehensive coverage.
Massachusetts is usually lumped together with zero deductible windshield replacement states because you can choose a zero deductible for your glass coverage. The Massachusetts Division of Insurance told us that there is not an actual regulation or law in their Commonwealth that requires the waiving of the glass deductible if you have chosen to have one with your MA car insurance policy.
As you know, comprehensive insurance covers glass, vandalism, theft, animal hits and fire damage. The glass coverage portion in Massachusetts has always had a variety of deductibles to choose from or as a policyholder you can opt to not have a glass deductible at all. When the Commonwealth's Division of Insurance was in charge of setting all car insurance rates, pre-2008, the glass breakage deductible was mainly set to zero for policies.
Since April 1, 2008 Massachusetts has allowed auto insurance companies to set their own rates (though filed and approved by the state) and offer various coverages. So many of the car insurance carriers who have just started doing business in MA the last couple of years, such as Geico and Progressive, include a glass deductible which allows them to give you a cheaper car insurance rate quote.
The MA Division of Insurance told us that with the exception of the car insurance companies who have just started doing business in Massachusetts; most other insurance carriers will default to having no glass deductible as it was prior to 2008. So while it is not state law that your windshield deductible is zero in MA until recently it was most common for your policy to come without a glass deductible for your Comprehensive coverage.
You should check your car insurance policy to see if you have a glass breakage deductible and call your auto insurance company if you find that you have a glass deductible but don't want one. If you choose to have a glass deductible it should save you a little on your car insurance rates.
The Massachusetts insurance regulator wanted us to remind you that the Comprehensive deductible is never waived under any other circumstances except as it applies to glass if you have a zero deductible.
Under MA General Laws (MGL) Chapter 90, Section 9A you are required to have safety glass for a windshield in your motor vehicle. Here the law basically says that a motor vehicle shall not permit the same to be operated with partitions, doors, windows or windshields of glass unless such glass is safety glass.
Safety glass is required because it is designed to minimize the likelihood of personal injury from its breaking or scattering when broken. This section of MA law does not apply to motor vehicles manufactured prior to January 1, 1936.
As for which law you can be cited under for a GPS on your windshield, it is MGL Chapter 90, Section 13. Here it says that no person, when operating a motor vehicle, shall permit to be on or in the vehicle or on or about his person anything which may interfere with or impede the proper operation of the vehicle or any equipment by which the vehicle is operator or controlled.
With this MA law it is up to the discretion of a police officer to decide if a GPS or anything else on your windshield is impeding your operation of the vehicle in their opinion and ticket you. From what we have read this offense typically comes with a $25 fine (other court costs, fees and surcharges may be added to it for a higher total amount due).