These are the domain of the freeway bloggers, a breed that have invented a tangible concrete and tarmac version of the internet to make their feelings known about George Bush. The messages, posted from overpasses, bridges and verges, are short, pithy and very, very rude.
How many of these bloggers are out there? No one really knows. Who are they? Mainly, it would seem, young men of a mildly anarchic disposition, with a message to get out, a modest talent for gymnastics and a pronounced taste for the adrenalin rush of their trade.
Are they breaking the law? Perhaps, though it's hard to argue that anti-Bush ranting is any more distracting to drivers than the raunchy fashion ads, local TV station posters and the other beacons of rampant consumerism that adorn every US highway.
These advertisers have to pay for the privilege of course - but what about that hallowed first amendment of the US Constitution, guaranteeing free speech and free expression?
Nor is the technique illegal. Back in that distant 18-month period of unalloyed patriotism between the 11 September attacks and the first adrenalin-fuelled days of the Iraq war, America's highways blossomed flags, diatribes against Osama bin Laden, and myriad calls to back the troops.
Now the politics has changed, and the messages have a darker ring. Next to an old sign bearing the message "Support our troops", a freeway blogger has added his suggestion as to how this might be best achieved: "Impeach the murdering bastards who sent them to die for a pack of les."
Another notes: "No one died when Clinton lied." Another cuts to the quick of the CIA leak scandal lapping at the President's top political adviser: "We support Karl Rove," says the message on the banner, signed "Americans 4 Treason.org"
Whether they are having a effect is debatable. Approval ratings for Mr Bush and his handling of the war are sliding to record lows - but the 1,800-plus US soldiers killed in Iraq, the 10,000 seriously wounded, and a seemingly unquenchable insurgency surely have a lot more to do with it than the musings of these 21st century political graffiti artists.
Unarguably however, freeway blogging is a highly efficient means of expression. "A blog takes me about seven minutes to trace and paint, six seconds to hang," says one practitioner. The materials - cardboard or cloth and paint- cost only a few dollars, and affixing them is also pretty simple.
According to one set of instructions posted on the internet, smaller signs should be placed against fencing and strapped in position with strong bungee cords. For larger signs, coat hangers as well as duct tape are recommended. The hangers should be taped to the top of the sign and then twisted around the fencing, before being fastened with the bungee cords.
And don't worry about the fencing obstructing the view. As long as the letters are six inches high, a sign will be perfectly legible. As for location, anywhere (almost) goes. Not just overpasses and verges, but "anything you can see while driving is a place you can put a sign", the instructions advise would-be bloggers.
"The more difficult it is to reach, the longer it'll stay up. Tens, even hundreds of thousands of people can drive by a sign before one of them takes so much as five minutes to take it down. Apart from actual prisoners, you won't find a more captive audience than people in their cars." Some of the signs disappear in minutes. But others stay up for months.
As a general rule, another blog-artist comments on the website www.freewayblogger.com, the larger the sign, the faster it comes down. "The most effective signs I post are small reminders along the peripheries of the freeway such as 'The war is a lie', or 'Osama Bin Forgotten'."
The spoilsports who take them down are, he presumes, "cops, highway workers and Republicans". But who cares, in the easy-come, easy-go world of the freeway blogs. "So long as you can keep putting them up, it really doesn't matter."
In a way, moreover, the medium is even more effective than the internet from which it draws its name. Political cyberspace is divided into ghettos of the left and the right - but as an aficionado puts it, "When you put something on the freeway, you get everybody."
And on the jammed California freeways where the art form was pioneered, everbody means a lot of people - tens, even hundreds of thousands of commuters on an eight-lane highway, all with no choice but to read these roadside political statements. For Republican drivers, it must be hell. But for the freeway blogger, life doesn't get any better.
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