I find it most helpful to start any problem like this by drawing a diagram. The most useful perspective to consider is that of an observer on the sidewalk. Take a 2-dimensional cross-section of what she sees. Such a drawing will include a triangle. The bottom edge of this triangle represents the letters, so that the two vertices of this edge are the top and the bottom of the letters. The last vertex represents the driver's eyes. You can then add the other information you have. For example, you know how far away the driver is from the lettering as measured down the road.
Once you have this picture, you can use trigonometric / geometric identities to find relevant lengths and angles. An important question to ask yourself is whether you have enough information given to you to find what you need. Are you comfortable using trigonometric identities and laws?
If this description of the picture is not clear, please let me know and I will attach a sample diagram.