My friends have a Red Grand Caravan V6 Auto with about 90k miles on it at least and I managed 29 on the highway with just the tires pumped up to 40psi. It does suck gas at idle 0.6gph as I recall so you have a loosing situation there - maybe jack up the wheels and check for bearing and brake drag would be a good start and wheel alignment too - uneven wear in the front and the rear tires. Get it parked the easy way out when the engine is warmed up so when it is cold you start and drive away. That is the first big hit you take when the engine is cold. Get it to upshift asap when cold and maybe try some Torco GP7 in the gas.
With the caravan I'm capable of getting 20.x on even 1 mile trips. Since you don't have a lot of opportunity for tricks/getting the engine warmed up etc. the critical factor is how many stops. I don't turn the car on in my drive until I see that I won't have to sit idling at the corner.
And coasting/not coasting is as much about timing making a turn or timing cross traffic as for the direct coast benefits. The first "mistake" where I pull to a safe stop and then can't immediately continue will bring it down to 18 with a quickness, and two bad stops will make it 16.5.
Is the unsafe bridge crossing a short distance from your house? Can you park on the other side of the highway and bike from there? It'll be a shorter, crummier mpg trip, but maybe you can still save half your gas.
My wife gets baaad mileage on our Ody in her regular short trip driving. Like 17 mpg. We get 28 mpg on the freeway, though. Anyway, she doesn't drive for FE. She holds the gas until the second she switches to pressing the brake. Ugh.
Yeah the only thing I could say is wrap the exhaust up to the CAT and O2 sensors and see if you can lower the idle setting when cold - those things race like crazy when cold for no reason. Put a little GP7 in the gas and maybe some STP gas treatment and Marvel Mystery oil too. Burn less fuel and lubricate the friction will get you better gas mileage.
I coast in the city and on the interstate. While coasting and at stops I put the van in Neutral. It usually brings down the consumption to .5gph at a stop. I would consider parking my van over the bridge but Southern Mississippi isn't a place where you want to leave your vehicle unattended for too long. I doubt I have dragging brakes or an alignment problem because the van only has 2k miles. I really want to get a scooter but the wife says it's too dangerous, but she is ok with me riding a bicycle (does this make sense?) I've considered buying an EV bike kit or an EV bike but they cost about the same as a cheap used scooter.
Four miles to work...what an ideal situation for a plug-in electric car...if you can get one that is fast enough. Even if you can get up to 5MPH under the speed limit, you shouldn't hold traffic up too badly in 4 miles...however, if its uphill, all bets are off. Hope you're electricity is generated by hydro or other renewable sources...let it not be coal.