Signal velocity in PCB traces
The rules of thumb given in the previous answer are good enough for many designs. But I want to add one additional thought.
The velocity factor is basically going to be the inverse square root of dielectric constant (\$\varepsilon_R\$ or \$D_k\$) of the material the electric field around the transmission line travels through.
For stripline, this means it's essentially the dielectric constant of the circuit board material.
But for microstrip, it will be an average of the board material's dielectric constant, and the surrounding material (usually air), weighted by the proportion of the electric field that travels in each medium.
That means that if your design has both microstrip and stripline, signals in the microstrip will travel at least slightly faster than signals in the stripline.
As pointed out in the comments, it's worth mentioning that the dielectric constant of PWB material can vary in service due to factors such as operating frequency, temperature change and moisture absorption. There can also be variation at manufacturing time due to factors such as etch-back of the traces and alignment of the traces relative to the glass fibers in the dielectric.
If I have a trace that has a width change, then there will be a characteristic impedance change, and that would cause partial reflection of the incoming wave. But there is no energy absorption anywhere, so the only cause for a partial reflection can be a change in the wave's propagation speed, right?
No, that doesn't follow.
If there's a discontinuity in the characteristic impedance, then you need a different ratio of current to voltage in the line for the incoming signal and the forward-propagating signal. That means to satisfy KCL and KVL (or, if you want a more mathematical explanation, to satisfy the boundary conditions) at the place the two geometries meet, a reverse travelling wave has to be generated. Then the current to voltage ratio on the incoming line at the junction can be made to match that on the outgoing line, and all is right with the universe.
The difference in propagation velocity on the two lines, if there is one, isn't important here.
If you consider the PCB trace as a lossless transmission line, the characteristic impedance \$Z_0 = \sqrt{\frac{L}{C}}\$ but the velocity factor is inversely proportional to \$\sqrt{L\cdot C}\$ (where L & C are per unit length).
So it should be possible for the velocity to change without the characteristic impedance changing, but it would require two things to change simultaneously.
It is true that if you only change the capacitance (or permittivity) then \$Z_0\$ changes proportionally to velocity factor. More in this 1965 paper.
Electromagnetic waves travel in a dielectric medium. In theory, the propagation speed depends on the relative permittivity and the relative permeability of the dielectric medium that the wave is traveling in. For all practical materials, the relative permeability is 1 so typically we ignore that, and say that speed depends only on the permittivity of the dielectric.
The formula is V = C/sqrt(epsilon)
Where V is the propagation speed, C is the speed of light in a vacuum, and epsilon is the relative permittivity. Typical circuit boards are made from a glass fiber epoxy composite called FR4.
The relative permittivity of FR4 is around 4, but this can vary with frequency and temperature.
However, for a trace on an outer layer, the dielectric is partially air and partially FR4. So for traces on an outer layer, usually an effective permittivity is calculated that tries to average the effects of the two different dielectrics. Since air has a much lower permittivity, outer layer signals are faster than inner layer signals. You can say that they will be roughly 15% faster as a rule of thumb. A detailed calculation can be made using formulas developed specifically for this purpose. But all the details need to be supplied (trace width, thickness, distance to reference plane, exact permittivity of PCB material, etc).
For inner layer traces, the speed is approximately half the free space speed. As previously noted, the permittivity in circuit board is a function of temperature and signal frequency. This is something you have to get from the FR4 supplier or the board fabrication house if you want to be extremely accurate.
FR4 is not the only choice of material. There are others, some specialized for high heat and some specialized for high frequency, etc.
Outer layer traces are typically modeled as microstip transmission lines. And inner traces are modeled as stripline transmission lines. Searching using those terms may be helpful in further research.