Wednesday, October 19, 2011

How wide will the Tappan Zee Bridge be?

There's something confusing going on with the design of the Tappan Zee Bridge replacement. I looked into this issue specifically, because I was disturbed in 2008 when the State DOT decided, without any public discussion, that the bridge would be ten lanes wide, with two of the lanes being "HO/T" or high occupancy/toll lanes, which could be used by carpools or by solo drivers paying a higher toll.

This represented a significant increase in the car capacity of the bridge from the current seven lanes. In addition to adding a peak-direction lane and two off-peak ones, it would have widened the lanes from eleven feet to twelve or fourteen, increasing capacity by allowing drivers to go faster without increasing their risk of crashing.

This increased capacity would promote residential and job sprawl at both ends of the bridge, but this was never discussed. It was simply announced on page 21 of the Scoping Update that the bridge would be ten lanes, and none of the Alternatives included anything different.

The build alternatives 3, 4A, 4B, and 4C include a number of common elements. The fundamental differences between the alternatives are the transit modes. The common elements include the following: ... A River Crossing with two HOV lanes, eight general purpose lanes, shoulders and a full-length pedestrian/bicycle path linking Rockland and Westchester.

I've been poking around through the old study documents I can dig up, but I haven't yet found any evidence that there was ever any discussion of how many lanes to put on the replacement bridge.

I've seen estimates of the width of the current bridge that vary from 82 feet to 90 feet, but let's assume it's 82 feet. It contains seven lanes of traffic with no shoulders. That comes out to an average width of 11.7 feet per lane. Considering that one of the goals in the Scoping Packet is "Providing for standard, 12-foot traffic lanes," it would seem that the current bridge is not that far off. The biggest problem is that it doesn't have shoulders for anyone to stop in if they break down.

The Journal News and many other newspapers are reporting that the current plans for the bridge call for it to be eight lanes wide. The scoping document tells a slightly different story, though:

Minimum Width

NYSTA would maintain traffic flow across the Hudson River to the maximum extent feasible, even if one of the two bridge spans must be closed. To provide adequate capacity for such short-term traffic operations, a minimum of seven highway lanes would be needed across the river. In order to accommodate this service redundancy, the road deck would need a minimum width of 82 feet to provide for seven, temporary highway lanes in the event that one structure would be inoperable. This minimum dimension would be provided on both of the new spans to achieve this desired feature of the new crossing.

At present, bicycles and pedestrians are prohibited on the Tappan Zee Hudson River crossing although there are existing multi-use trails near the bridge on both sides of the river. To maximize the public investment in a new crossing, the New York State Department of Transportation (NYSDOT) and NYSTA would provide a shared-use
(bicycle/pedestrian) path across one of the spans of the replacement bridge. To meet current design standards for the path and to provide adequate separation from traffic lanes, the Replacement Bridge Alternative must provide a minimum of 12 feet of additional width for the shared-use path (including the path, barrier, and a safety buffer); however, 14 feet is currently planned.

To meet these operational requirements, the EIS will consider a Replacement Bridge Alternative with two spans. The span that includes a shared-use path would be 96 feet wide, and the span that does not include the shared-use path would be 82 feet wide.

If you're paying attention, you'll note that the new bridge will have two spans, each of which can carry the same amount of cars as the current bridge. That means that the bridge could conceivably carry fourteen lanes. What's up with that?

It's true that the Governor has been complaining about the "seven narrow lanes and no safety shoulders" under the current configuration. So we would imagine that the new bridge would have at least twelve-foot shoulders and fourteen-foot lanes, and we still get ten lanes.

Ten lanes might not be so bad, because two of them could be used for buses right away. It's possible, however, to squeeze twelve lanes onto the bridge and still have twelve-foot lanes and an eleven-foot shoulder on each side.

The main thing that I want to know is, what's to prevent the bridge from being used for fourteen lanes? Right now the Thruway Authority could dramatically decrease the number of crashes by reducing the number of lanes on the current bridge from six to seven, with either two six-foot shoulders or one twelve-foot one. They don't do that because the politicians on either side are insisting that car throughput is more important than safety. The current level of congestion, they say, demands seven lanes.

If the "eight-lane" replacement bridge fills up, as Kate Slevin predicts, there will be new demands to put another lane or two on it. If the bureaucrats couldn't say no to increasing this bridge to seven lanes, why would they say no to fourteen?


jazumah said...

I think the notion of full car lanes is going to end on many tolled facilities soon enough. The economic collapse that is hampering bridge financing is also reducing traffic on tolled facilities. I don't think more than eight lanes will be needed for the new bridge. Furthermore, there is nowhere for the extra lanes to go on each side.

Zmapper said...

I heard that the Tappan Zee was 90' from inside girder to inside girder. If so, then they could have 8 lanes during rush hour. It would look something like this:

12' truck/car lane
10' car only lane
10' car only lane
12' bus lane
2' post only median (like the Golden Gate bridge)
12' bus lane
10' car only lane
10' car only lane
12' truck/car lane

Outside of Rush Hour, 6 lanes could be provided with a bit more space for maneuvering. I suspect that the speed limit might be reduced to 35mph or so because of the narrowed lanes, though I dought the traffic is moving any faster than this already.

Jeffrey Jakucyk said...

These highway standards are really insidious. Keep in mind that once you have more than two travel lanes each way, you need full right AND left shoulders. There's a large bridge replacement going on north of Cincinnati (the Jeremiah Morrow bridge over the Little Miami River gorge) that's a perfect example. This 1960s era bridge should be replaced in no small part because it's the same fracture-critical under truss design as the I-35W bridge in Minneapolis.

Here's where it gets crazy though. The old bridge (actually two separate bridges) has no shoulders. It's just two lanes each way with an extra foot or two on the sides. If the bridges were rebuilt in the same configuration, you'd have a full right shoulder, and a little more buffer space to the left, probably 3 feet. So a simple as-is rebuild to modern standards, assuming no change in the lane width, means the pavement width increases by 50%+ with no increase in capacity.

That's not where the insanity ends though. They're building the replacement bridge with room for 3 lanes each way. That means now we have to have full right and left shoulders on top of that third lane that's probably going to be blocked off. So a 50% increase in capacity yields a 150% increase in pavement width over the current design. This doesn't factor increased lane width either.

It's really maddening to know just how much space and money is taken up by shoulders. A modern 6 lane highway has a total of 10 lanes, that's 40% of the pavement you can't even drive on. Is it any wonder these highways and their bridges are so horribly expensive?