Having driven both routes to Placenica dozens of times, I would certainly concur about:
1) Don't drive at night if you can avoid it. Besides the other dangers mentioned, fog often rolls in at night on the Hummingbird making driving very dangerous. Also, if it is raining, it is incredibly difficult to see where you are. Last time I went to Placencia, a few weeks ago, I drove at night in the rain and missed the turn-off to Placencia, even though I am familiar with the road. Just drove right past it due to the rain and construction on the road. It was several miles before I figured out I had missed the turn to Riversdale.
2) Road signs from Dangriga south are no longer very dependable. Road construction has torn a lot of them down, and the storm damaged others. There are no longer mileage markers for most of the Southern Highnway.
3) I have found the maps offered free by car rental companies are not very helpful. Some are many years old. The ITMB map (the new 2001 edition) is the best general map available. You can usually find it at the international airport, though it may be an older edition and it sometimes costs more than if buying it before you go (US$8.95).
4) The coastal highway is a bit shorter, but the Hummingbird is much more scenic and a since it is all paved is much less stressful.
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