I have the deed and the pertinent description of this prescriptive
easement is: "a strip 16 feet in width, for the purpose of roads and
pipe lines..." This easement is an unimproved alley which is not
maintained by the County in any way. The easement is used for ingress
and egress to a property behind my land.
Can the owner of a prescriptive easement or anyone else make
improvements or work on this easement located on my land WITHOUT MY PERMISSION?
STATE OF CALIFORNIA.

For $2, you may have to settle for just a free comment or two.
I am no lawyer or professional, and as you can read in the disclaimer
below, this is no legal or professional advice.

This is an excerpt from one site:
"In Otay, the Court defined the scope of a prescriptive easement as follows:

?The scope of a prescriptive easement is determined by use through
which it is acquired.  [Citations omitted].  [T]he only limitation [on
future use of a prescriptive easement] is imposed by the use
made?during the statutory period.?  [Citations omitted].  Once a
prescriptive easement has been acquired, the location and extent of
its use is determined by the use experienced during the prescriptive
period.

Otay at 1047.

The parties? respective rights to an easement were defined in
Mehdizadeh v. Mincer  46 Cal. App.4th 1296, 1307-08 (1996) as follows:
 An easement defines and calibrates the rights of the parties affected
by it.  ?The owner of the dominant tenement must use his or her
easements and rights in such a way as to impose as slight a burden as
possible on the servient tenement.  [Citations omitted].  The owner of
the servient tenement may make any use of the land that does not
interfere unreasonably with the easement.

Unlike adverse possession, when the adverse user claims an exclusive
right over the disputed property, it is well established law in
California that one cannot acquire an exclusive prescriptive easement
over residential property.  With an easement, the property is
?shared?.  Id. at 1308, Silaccci, supra , 45 Cal. App.4th  at 564."
http://www.bullivant.com/media/showstoryidea.asp?Show=3186

Scroll down to pages 22 and 23 on this site for a discussion of
improvements that seems similar to your situation:
http://www.californiasurveyors.org/files/calsurveyor/Cal%20Surveyor%20143_2.pdf

From the excerpt, it seems clear  - to me -  that your neighbor cannot
make any improvements on the area of your property, just as you cannot
obstruct his access within the terms of easement ("...for the purpose
of roads and
pipe lines...").  

Personally, I expect that there is a mutual interest and maybe
responsibility to maintain the alley in its original condition, that
is, there wouldn't be much argument against his filling in potholes.

I hope that if this information is incorrect, someone will quickly correct it.

I'm not sure what you're asking here.  A prescriptive easement, by
definition, is not part of the deed.  It is acquired by its use of a
third party in a manner that is hostile to the actual landowner.

After the passage of the requisite amount of time during which the
hostile use(that is, use without the landowner's permission) is made
of the land by the third party, a prescriptive easement is formed, but
it is not noted on the deed.  After the formation of a prescriptive
easement the third party can therefore continue to use the land (in
this case to enter and exit) even if the landowner now objects and
brings and action to stop the trespass.

If the deed contains an actual statement as you describe above, then
the easement is probably termed an expressed easement appurtenant. 
Regardless of the type of easement, the use cannot exceed the scope of
the easement including foreseeable future uses.

When you ask, "Can the owner of a prescriptive easement or anyone else make
improvements or work on this easement located on my land WITHOUT MY
PERMISSION?" the answer really depends on the what work or
improvements are being made.  If the alley has fallen into disrepair
and the work is necessary for the easement to continue to be used
(i.e. fixing potholes), the answer is probably yes.  If the alley, as
is, permits the easement to be used as it was envisions, then any
improvements that use the land (even if the improvements are
beneficial - i.e. planting of flowers or placement of lights) in a
manner that exceed the scope of the easement, then the probably
cannot.

If you post what use is being made, perhaps you might get a clearer answer.

Note - I am not an attorney and therefore the information provided
should not be considered legal advice.