@Bronwyn
I reduced my ins cost by about 50% by going $100/300kvliability only on a car & truck that are only worth about $10-15k each. I also have a $2M personal liability umbrella policy
I was already getting a "low mileage" discount for driving each less than 2.5-5 miles/yr.
I can afford either pay for collision repairs myself or to scrap the vehicle & buy another, if I can't get the other party's insurer to pay for any collision damage.
Not a good idea if you can't.
Also note that if you go liability only, your insurer will have no interest in acting on your behalf to help you collect damages to repair your vehicle from the other party or his/her insurer because they will have no responsibility to pay for such damages for which they would want to be reimbursed. You'll have to do this on your own.
Increasing collision deductible decreases your premiums only marginally but any little bit helps provided you have enough $ to pay the deductible when & if necessary.