Florida 1950s & 60s Partial Cloverleaf Interchanges

[Fred Defender]

I'm rather new here so forgive me if this topic has been discussed before. I'd bet that it has, but a quick search did not turn anything up.

What is the story behind Florida's fascination with partial cloverleaf interchanges on interstates designed/constructed in the late-50s/early-60s? I-10 between Lake City and Jacksonville and I-75 from Alachua to the Georgia line both have a number of these types of interchanges. True, in some cases there are/were impediments (i.e.,I-75 adjacent railroad tracks) which preclude a full diamond interchange. I-75 Exit #399 (US441) is one such interchange (there used to be a rail line parallel to 441 south of the exit). But there are many such PCI's where no such impediment existed. I-10 at Chaffee Road and at FL121 are two examples. I-75 Exit #'s 439 & 451 are two more. I guess PCI's are, arguably, a more efficient use of land.

What really puzzles me is this:

Why would engineers design these interchanges with EXITING traffic making the cloverleaf? I live near I-75 Exit 404. It's another one. The topography at 404 may have dictated the design so I'm not going to b***h about that one. But making exiting traffic decelerate rapidly, especially in inclement weather, can be downright dangerous. In other words, if you're an engineer and you think that PCI is the way to go, why not reverse the design and make the ON RAMP the cloverleaf?

[NE2]

Better to have slowing traffic where people behind can see than cars entering the Interstate at slow speeds with limited visibility.

PS: this type of interchange is specifically called a folded diamond.

[Fred Defender]

Better to have slowing traffic where people behind can see than cars entering the Interstate at slow speeds with limited visibility.

PS: this type of interchange is specifically called a folded diamond.

I'm sure no expert. But I respectfully disagree. It is easier to start making a ~270 degree turn at a lower speed and finishing it at a higher speed -especially when the on-ramp begins straightening which allows for safe acceleration - than vice versa. Brake lights on a through-lane caused by slowing traffic entering a cloverleaf at 70+MPH (or, worse, being flung off the ramp due to the forces of physics) is not a good thing.

Again, JMHO. Guess that's why I'm not a traffic engineer.

Sorry for the improper terminology. I'm, obviously, no expert. Here to learn. Thanks.

[jwolfer]

I always thought the thinking was to slow traffic exiting the freeway to prepare them from driving on surface streets

[Fred Defender]

I always thought the thinking was to slow traffic exiting the freeway to prepare them from driving on surface streets

I guess it does accomplish that. I-75 Exit 309 in Sumter County is another one. Real estate is cheap there.

[jwolfer]

Chafee rd exit was just rebuilt a couple years ago. The clearance over the interstate was raised and I think that the turning radius is larger. Acceleration and deceleration lanes longer

[Tom958]

Also, with loop offramps, the directions that freeway-bound traffic should take would be more intuitive that they would with loop onramps.

To me, that type of interchange seemed like a good idea, mainly because the loop-direct formation would be easier to signalize,  and because the diameter of the loop ramp provides extra sight distance at the ramp terminals that otherwise would require additional right of way. I suppose that experience proved otherwise, though.

[Fred Defender]

This is the most glaring example. I-10 Exit #335, FL121, Macclenny. No topography issues which would have prevented a full diamond. The deceleration lanes are not that long; you really have to get on the brakes after exiting the through lane (unless you want to decelerate sooner and risk getting *ss-ended). I live about five miles east of I-75 off Exit #404 so I drive that one frequently. Same design. It was built several years later (1963) and the exit was constructed on a hillside of sorts. So I'll cut the engineers some slack there. Not so Macclenny (c. 1960):

https://www.google.com/maps/place//@30.2590384,-82.1223985,18.23z

Indeed, jwolfer...Chaffee Road is much improved.

[jwolfer]

This is the most glaring example. I-10 Exit #335, FL121, Macclenny. No topography issues which would have prevented a full diamond. The deceleration lanes are not that long; you really have to get on the brakes after exiting the through lane (unless you want to decelerate sooner and risk getting *ss-ended). I live about five miles east of I-75 off Exit #404 so I drive that one frequently. Same design. It was built several years later (1963) and the exit was constructed on a hillside of sorts. So I'll cut the engineers some slack there. Not so Macclenny (c. 1960):

https://www.google.com/maps/place//@30.2590384,-82.1223985,18.23z

Indeed, jwolfer...Chaffee Road is much improved.

What's odd is exit 336 for SR/CR 228 is a diamond. They seem to be from the same era. But there seems to a lot of accidents near macclenny. The folded diamond I think leads to some of the wrong way drivers.. Usually drunk or elderly getting confused at the entrance

[florida]

I-10 @ SR 121: CR 23A was there first. I-10 came later.