The new Baltimore is a nice place to live, but you would not want to visit -- not if you are inching through the grimy old Harbor Tunnel, that is.

For interstate travelers, the dread begins miles away as they steel themselves for the maddening Harbor Tunnel bottleneck that often forms miles outside of Baltimore.

But all that's about to change. Travelers on I-95 who scarcely glimpse Baltimore's restored town houses, its sparkling Inner Harbor or the growing downtown skyline will soon see a new side of the city as they whisk through its gleaming new Fort McHenry Tunnel.

The new eight-lane tunnel -- a massive $750 million engineering project five years in the making -- will open today after a 3 p.m. ribbon cutting, lengthy ceremonies and special motorcades, just in time for the Thanksgiving travel crush.

Dozens of state and federal dignitaries are expected to be on hand for opening ceremonies today to claim credit for the largest underwater road project in the history of the interstate highway system, one that came in under budget and almost on time.

The 1.7-mile tunnel runs beneath the Patapsco River, connecting historic Fort McHenry to the industrial area of Lower Canton and completing I-95 through Baltimore. It's made up of four two-lane tubes, with two tubes for northbound traffic and two for southbound traffic.

The tunnel, by its size and design, is expected to eliminate traffic tie-ups. The Fort McHenry Tunnel can handle 100,000 cars a day, nearly twice the capacity of the four-lane Harbor Tunnel, which will remain open and undergo staged renovation beginning next year.

The tubes of the Fort McHenry Tunnel are the largest ever built, allowing for walkways along both sides of the road and 12-foot wide traffic lanes. Designers hope the wide lanes will keep traffic moving briskly at the 50 mph speed limit.

To eliminate tunnel hesitation -- the instinct of most drivers to hit the brakes as they approach a dark hole in the wall -- daylight is simulated in the first 300 yards of the new tunnel by white pavement and high intensity lighting. The walls throughout the tunnel are covered with 8 million white, bathroom-style tiles that took a crew of 200 two years to install.

There is much more to the tunnel than drivers will ever see. Housed above, in buildings at either end, for example, are dozens of electric fans, each two stories high. Half the fans will draw exhaust fumes out of the tunnel through ceiling vents; half will pump fresh air down into the tunnel through vents at curb level.

But the heart of the tunnel is in its control room, a vast, space-station-like arena where tunnel police can monitor everything from air quality to traffic speed. There are banks of switches and dials, and on one side of the room, 64 television screens displaying every square inch of tunnel.

"It looks like if you press the wrong button it might take off," said tunnel administrator Thomas Fallon.

If control room instruments show high levels of carbon monoxide in the tunnel, police can turn on more fans. If speed sensors imbedded in the tunnel roadway show traffic is moving too slowly, a computer-controlled sign will tell drivers to speed up. The television monitors make the tunnel a steel trap for speeders and drunk drivers, who can easily be picked off by police as they emerge.

"They've got so many eyes on them, we can just intercept them," said tunnel police Lt. Norman Boskind.

Police can also flash signs to motorists telling them to turn on their radios, then broadcast emergency or public service messages directly to them. Special receiving equipment allows drivers to pick up both AM and FM frequencies in the tunnel.

The most important function of the tunnel's state-of-the art monitoring system is the quick response it allows police and rescue officials in case of accidents. Authorities can see what is happening immediately and be there in minutes.

In the Harbor Tunnel, an accident or breakdown can bring traffic in either direction to a dead stop for hours. Because the new tunnel has two separate tubes going in each direction, traffic in the other tubes will keep moving even if one tube is blocked.

Using federal funds, the city began construction of the tunnel in May 1980, with a targeted completion date of Labor Day 1985. Its final cost, estimated now at about $750 million, is well under the projected budget of $825 million. The city will now turn the tunnel over to the state, which will float bonds to repay the federal government 10 percent of the tunnel's cost. The state will eventually recover that money in tolls of $1 for cars.

From the beginning, the tunnel project was enormous. From an engineering standpoint, no other like it has been built. You can't see the light at the end of this tunnel until you're upon it, for the tunnel itself is curved, horizontally and vertically.

"As transportation engineering goes, this is kind of the ultimate; this is an engineer's dream," said project engineer Kenneth Merrill.

Because of the horizontal curve, no two of the 32 steel barge-sized tube segments are exactly alike. Merrill described each segment, which is shaped of two adjacent tubes, as looking "like a giant pair of binoculars." They were made in Port Deposit, Md., floated 45 miles down the bay to the Baltimore Harbor and then sunk into a giant trench that had been dug in the harbor floor.

The fill from the trench was used to form the nearby 147-acre Canton-Seagirt landfill, which is someday planned for use as a shipping terminal.

At the peak of construction activity in 1982-83, about 3,000 people were at work on the tunnel. Barges lowered tunnel segments into the trench, scuba divers aided by huge hydraulic jacks connected the segments, and workers built the roadway inside the completed sections.

By this month, there were only about 200 workers left, doing such last-minute tasks as debugging the computer system. Toll-takers to staff the 24 new booths had been hired and the tunnel police force beefed up.

But there is another big project ahead: The 28-year-old Harbor Tunnel, already an ugly stepsister, is going to get a two-year, $40 million facelift starting next spring. Each of the tunnel's two tubes will be closed for about a year as new roadway is laid, lighting improved and those dingy yellowed walls covered over with white tiles.