If you are able to edit your street data and re-build the network dataset, then you can add a new travel time fields, calculate the new travel time values according to your needs, add a new impedance attribute to your network dataset from these fields and rebuild the network. Now use the new impedance attribute in your analysis.
If you are unable to edit, another approach can be to use barrier polygons with scaled cost, not restriction values in the service area layer. You can create a polygon over your study area and set the scaled cost to 0.9 and all the streets that fall in that barrier polygon will have their travel times reduced by 10% and thus your vehicle can go further. You can adjust the cost to 0.85, etc. as needed for your analysis. The only issue might be that if you have a lot of streets that fall inside the barrier polygon then it can be slow to load the polygon during analysis.
Jay Sandhu
