Gado

Do you keep your bike accessories on the bike while it is on the trunk/hitch/roof rack? Things like pump, wireless computer, front and rear lights, saddle bag, etc. If they can stay in place while biking, they should be able to stay while being transported, right? What if you have to make a quick stop to a gas station or LBS?
Currently I take everything off but maybe I can save some time by leaving all or some accessories on there.

clarkbre

I take them all off, throw them in my rear rack bag and then re-accessorize when I get to my destination. I dont' care so much if someone steals that stuff. But, I (and the driver behind me) would care if something came flying off my bike potentially damaging anything behind me. And, realistically, by having to re-accessorize your bike at the destination allows you to do a quick once over before you get on and ride.

Monster Pete

My lights are bolted to the bike, so I don't have much choice. I once drove my bike somewhere with the water bottle still on if that counts

IthaDan 

Bottles, lights and computers always come off. Panniers too.

It only takes a second, I don't see a point in risking it.

RonH

For long hauls at highway speeds I remove everything. For short trips I remove only the things that can come off/bounce off easily.

redeyedtreefr0g

I haven't ever thought to remove my stuff before.

I have a cup holder on my handlebars, and previously a cheap Bell Radian headlight mounted to my stem (I swapped to get the angled rubber from the seatpost rear light).
On the seat tube I have my air pump mounted so that it is sandwiched between the tube and the rear fender.
On the rear rack I used to have a plastic crate and the rear light bungeed beneath it.

NOW, I have a bottle generator light system instead of the battery lights. I added a home-made pannier instead of the green crate. It ties onto the side of my rear rack with ribbon in five places. The generator for my lights just broke a few days ago and in less than 60 seconds or so, I mounted my flashlight to the handlebars with a length of old tire tube.

I would take off the flashlight now to transport my bike by car since it isn't actually bolted on, but I was very surprised how stable and secure it was when I rode with it for the first time on Friday. I'm not worried about losing my flashlight during my commute. Usually my water bottle stays put in the cup holder very nicely too, but for long trips by car I would not leave it in there. Usually my bike is only on a car because I needed rescuing due to ornery flat tires. My new thorn-resistant tubes should make that a thing of the past though

Berg417448

I leave everything on the bike all the time. I've always done it that way. Never saw a need to remove anything.

Altair 4

During local transport (under 5 miles) the bike computer and PB Blaze headlight come off but the water bottle, air pump, and saddle bag stay on. On longer drives, the water comes off, too. Everything else stays on. If I'm using my trunk bag, I'll take it off; it presents a little too much wind resistance at highway speeds and no use tempting the fates.

Ozonation

I remove all detachable items, and would make sure all bolted on items are really secure. Nothing like a good bump on a road to shake something loose, hit somebody and something, and then be found liable for it or feeling guilty for an injury.

Keep in mind that on a car you're reaching speeds much higher than on a bike of course, and bumps and swerves tend to be more severe. A panier loaded with some tools, weighing 5 kg and falling off your bike at 20 km/h on a fairly uncongested bike path is one thing; falling off at 100 km/h onto a highway with bumper to bumper traffic could be a recipe for a disaster.

Besides... how long can it really take to reassemble things, particularly if they're meant to be easily attached/detached? Nevermind exposure to the elements: rain, wind, mud, dirt, other flying objects. And safety and peace of mind! I don't know about others, but I tend to just stuff things into paniers and bags, forget about them, and if they drop out or are stolen, well, that would be a bummer, eh?

eja_ bottecchia

I remove everything except the seatpost mounted rear light.

The way my bike is set up, it takes a minute to put it all back on.

myrridin

Those items that are securely attached to the bicycle, such as bottle cages, seat bag, pump, etc.. are left on. Those that can detach themselve, such as the bike computer and water bottles are kept in the trunk. I use a roof rack.

DEK

Saddle bag and computer (basic cycle-computer) stay on the bike. The saddle bag has 2 straps holding it on and the computer snaps into the holder so I see no need to take them off. I do take the water bottles off. I don't carry a pump and don't have any lights.